@Tilllly a couple of suggestions though of course what works for me may not work for you.
Seconding dietitian and if you can find a dietitian who works with cancer patients it can be really helpful.
Hydrating with electrolytes, but in proper proportion is helpful, coconut water while nice hasn't got the right proportions but a commercial solution will, I just don't know the UK products. Dioralyte sort of thing? We have ones called Hydralyte and Pedialyte here and I always have them in the house to sip on as I'm prone to dehydration.
Also seconding eat what you really fancy. My onc said I don't care WHAT you eat I just care THAT you eat.
I rely a lot on soup, it does go down easily and sometimes it's all I fancy and easier to eat than a plate of something solid. I used to have ice cream but now on oxaliplatin I have cold sensitivity so I have yoghurt as I can warm it slightly in the microwave. There's drinkable yoghurt too. Or as has been said the shakes. Slurping is good, I agree!
For protein you can mix protein powders into things or nibble on your favourite protein, chicken, cheese, nuts or whatever. You can make smoothies with almond butter. Buy the things you really like even if pricey, I like Boursin for instance and can eat it on its own or on a cracker. Or hummus or some kind of potted meat, or peanut butter and jam.
And light things, fruit if you like it, fresh or tinned for example.
But the most important thing is to enjoy what you're having. Serve yourself little portions so you're not trying to force a lot down at once and then you can either have seconds or if you're then sick of the meal switch to a pudding, biscuits and tea or whatever feels right.