As n update, having driven the car a bit more, I'm 90% sure the issue is a failed lambda sensor - the one after the cat. I've spoken with Martin Smith at https://martinsmithmgspares.co.uk/ and he has one in stock so will go and pick it up. Hopefully he'll be able to plug in and read my fault codes whilst I'm there, as I suspect one of my engine temperature sensors is dodgy too.