I'm quite far into my PhD my letter from my supervisor didn't mention my end date. We found Nationwide wouldn't even let us get a quote as I'd be going onto a research post which again is fixed term. Natwest seemed accommodating and spoke to their underwriters especially for us. Obviously it is a gamble with a close end date but myself and my partner reasoned I could work at a supermarket and it would pay my half of the mortgage if we had a worse case scenario (fingers crossed we don't!).
I also work in a restaurant part time and earn an extra 5.5k a year which is taxable and has wage slips - so this really helped. We also have a 25% deposit so Natwest didn't want to turn that away.
If you do any teaching on the side - this is good to mention as will be taxable.

Thank you Matt!! Following your post myself and my boyfriend have secured a deal with Natwest.