So yeah, after months of topping up and carrying on I went to see blu73 last weekend and we did the timing belt, water pump and thermostat.
After a lot of headscratching, it looks like there's a hole in the radiator somewhere close to the bottom as we filled it back up after putting it all back together again and it immediately started dripping quite badly, however when we put the cap back on the reservoir it stopped. Chris equated it to holding your finger over a straw full of water.
So I've got a new radiator today, and will hopefully get that fitted this weekend.
Being on lockdown means I'm working from home, so at least I don't need to drive anywhere!