If you feel that you don't deserve to be doing a PHD then presumably you also believe that everyone else in your group does deserve to be doing one.

That's flawed logic right there. I guarantee you that most of your colleagues lose sleep at night over this exact same feeling. I didn't meet too many people who didn't suffer IS at some point. Look around you. Whether they are telling you or not, I reckon at least 7 out of 10 people that you can see have these doubts.
Part of the PhD mountain is learning to stop obsessing over things which don't matter. The word "deserve" doesn't belong in the world. Nobody "deserves" anything. Everything has to be earned. You are not alone in this.

You have chosen (along with the rest of us) to spend the next few years pushing yourself to the limit academically. By definition this means spending vast amounts of time outside your comfort zone. Your brain doesn't always like that and Imposter Syndrome is one of the many shitty ways it uses to try and force you to go back inside to the warmth of your comfort zone. To stay out in the storm takes real guts which is why lesser people don't manage to get a PhD, and prefer a safe career rather than trying to run their own business etc. Imposter Syndrome is a sign that your PhD is doing what you wanted it to do - stretching you to your limit, just like rain tells you that you are outdoors. Embrace it for that reason.

I used to annotate printed sheets of the lecture notes.

Mine was 2 hours long.

Ah that's bad news. Unfortunately you've learned a painful lesson here. Not much help now but you should have waited until you got that offer in writing before turning down the MC award. Not sure what to advise here.

Presumably minor corrections will be handled only by your internal examiner so unless you miss out a correction or your internal is a dick, you will be passed for full publication and graduation. I don't know of anyone failing after minors.

Do you have the 1450 offer in writing?

I'm not sure I understand the original post.
It is perfectly reasonable to be tested on both your work and the background theory behind it.
The idea is to prove that you know what you are doing, why you are doing it, that it is actually you doing the work and that there is sufficient work being done.
This seems fair to me.

Is your partner asking for support or are you giving it without being asked?
There are probably no words which will help if support is being given without being asked for.
From personal experience I prefer to sort things out myself. If my wife or anyone else constantly brought up my situation without me asking it would make things immeasurably worse and would probably ruin the relationship.
So my advice is to ask yourself whether you need to maybe say less to them and give them space to work things out for themselves.
I have to admit though that at 4 years it's going to be time to move on.

Finishing a Part Time PhD in less than 6 years is very ambitious.
I wouldn't expect 4 years to be achieveable at all and certainly not at the 5-10 hours per week that eng77 is talking about. Most PhD students struggle to finish a full time PhD in 4 years.

Happy to be corrected on either point but I think your expectations are a bit high in terms of duration. Realistically the PhD will consume almost every spare moment you have outside of your part time job.

Your age is definitely not an issue in the UK. I was almost 40 when I started my undergraduate degree and 48 when I finished the PhD.

I can't comment on costs unfortunately.

I'm actually surprised to hear this. I genuinely thought this would be a strange tactic but obviously if you have direct experience of this working then maybe that is the right path for you to go down.

Agree with TQ and eng77.
I would not encourage anyone with a 2:2 to undertake a PhD but your Masters dissertation will definitely trump that and you should be fine.

A collaboration would be a peer to peer relationship.
You would then be asking a peer to employ you.
That sounds like an odd strategy to me although I can see why you are thinking about it.

If this was me, I would either interview them and take a chance or I would attempt to bypass the entire thing by trying for my own funding on an idea which was exciting and ambitious.

I think your original question made sense. Reading back, I just managed to completely misunderstand it. I was also partly answering rewt's point.

OK I'll have another go.
I can't see how the size of the group would tell you that.
The only way to know is to interview the supervisor directly as though you were hiring them for the job.
Think of all the bad scenarios you want to avoid and then ask a question to directly find out if that's what you'd face.
I would ask directly what their expectations were of me and others in the lab. I'd use that to start digging as deeply as possible. For example, is this person expecting a certain number of publications per year, to publish only in certain impact factor journals, etc. If any of those questions were answered with numbers I'd stop the interview and walk away because for me that would cross two red lines.

Other than interviewing them personally, I can't think of any other way of finding out if they are a good match for you. Most people don't do this because they are so desperate not to be rejected. That leads to the sort of trouble which could have been easily avoided. I certainly would never trust the opinion of another student or postdoc.

The main thing is that you took this into account before making your decision.
Eng77 was entirely wrong to tell you to "be grateful". You owe nobody anything and should do whatever you feel is best for you. Others may well have given their right arm to be offered that opportunity but that is their problem not yours. You can't spend your life toning down your ambition because others out there fail to achieve theirs. You earned the right to turn that position down. They did not. Their problem, not yours.
Good luck on your decision. I think you have done the right thing for the right reasons.

I can't see how the size of the group would make any difference.
I would advise you to think about what kind of career you want and then select the route which best suits that.
It sounds to me that you are looking at the wrong metrics to make a good decision in that respect.
For example, your supervisor's publication record and that of his previous students isn't helpful. It's going to be entirely down to you now. You might find you get lots of papers out or none, regardless of their previous output. There's no correlation between the two but you appear to be looking for and expecting some.
As for impact factors, you already know my feelings on that score :-D