Thanks for the advice people, I think I'm going to go ahead and quit though. In the day or so since I've decided this it's felt like a weights been lifted off my shoulders, I've felt happier than I have done in months and had several full nights sleep. My only concern now is how to tell my main supervisor since he was a nice guy and I don't want to burn bridges etc. Also does anyone know what the consequences of leaving are/the procedure i.e. will I have to pay any money back? Will I have to have multiple meetings with head of school/program etc or will I just be able to leave hassle free?

I am 20 months into my 42 month PhD project. My project is industrially sponsored and I was told from the beginning that they would have an influence on the project direction. My original proposal was accepted by my supervisor but 6 months in, my industrial sponsor changed its mind about some aspects of the project which meant all of my methods of analysis were now incompatible and the work I had done so far was now useless.

After spending 5 months trying different ways to adapt I was forced to have a major rethink, my co-supervisor strongly suggested a new route which he claimed was simple and easy. I had absolutely no experience from my degree in this new area but from what was described I thought it was the miracle fix and leapt on it, taking 1-2 months to read up on the technique. I discovered that despite his claims, there was limited literature for my specific application of the technique and that the way I would be using it was slightly different from the normal. My main supervisor had absolutely no knowledge/experience of this new technique so I was left with my co-supervisor taking the lead. He kept passing me off to his other PhD students whenever I asked for help and they could contribute little since I was using this adapted version of the technique.

I have spent the past 8 months attempting to get the technique to work to no avail. I have a yearly transfer report/viva in two months, I scraped through the last one by the skin of my teeth. At the moment I will be going in with no data and a load of excuses. There is an alternative technique which may work but I wouldn't see results until after the transfer report/viva.

At this point I would rather just quit since I am incredibly stressed, I'm losing sleep and I'm miserable. If I'm likely going to fail my yearly progression is it better to just quit now?