Can I ask for some advice again?
I have a table assessing 5 computer programmes according to their suitability for carrying out my analysis. None are perfect but one comes out streets ahead of the rest. Unfortunately this is not the programme I used because I only found out it existed two months ago. Should I admit that failing as one of the things I would have done differently in hindsight or should I try to justify why the programme I did use is best (it does have some advantages over no.1 prog) and risk the examiners pointing out that my choice was poor. Or can anyone think of any other way to write this up that minimises the fact that I mucked up?

Thanks for any suggestions.

I'd be inclined to be honest and say, @at the time of the analysis the chosen programme was the most suitable I knew of' and look into the newly found one as would have done differently. Saying that, I'm taking the honest approach as I consider blagging a risk. What if they KNOW it's not? You know it's not so your defense may not be as strong either as you won't believe in it! Is the one you used the second best?

I would agree with Skig that honesty is the best policy. That being said, you could describe what you did etc in your methodology and leave the [better] alternative until later on in the thesis when you address possible limitations. Then you could 'admit' that you were unaware of this computer programme until your data collection/research was well under way. You could also link it in with any recommendations for future practice.

That way you are acknowledging it but not labouring the point. It also could make an interesting discussion point in your viva when you get to that point.

Pragmatism can be a legitimate reason for using a particular method/approach/piece of kit.

I agree with the others. You could also add that for future work you might switch as there would be benefits to using this particularly programme but that it wasn't practical to make the switch earlier. As long as you didn't show the one that you used to be terrible then it's ok!

Thanks for your comments, I’ve written it as you suggested with the annoying new programme as a future revision rather than floundering around trying to fudge a reason.