Thanks Frus, it's helpful to have someone who's a bit more knowledgeable than me about these things!
At the moment I have more faith in their ability to get him a referral than our GPs. They were mostly helpful & encouraging & talked a lot about gathering evidence & having another meeting in 6-8 weeks to discuss how things are going. I'm in no great rush to get an official diagnosis but I feel like it would give me more confidence in the future if I could explain to people that, for example, a last-minute weekend away with us won't be a fun relaxing break because we'd spend all night trying to pin Calf into his bed, in a way that doesn't make us just seem like inept parents. I guess some people will assume that whenever we mention an aspect of his behaviour we struggle to control - those will be the same people who claim ASD is just a pseudonym for bad parenting. 
I can't imagine visual timetables would help Calf either, he'd probably just rip them off the wall... He does like to know what we're doing & when & gets upset if we change our plans without informing him. Timetables really helped my niece when she stayed with us though - I wrote out activities we could do & she helped choose what we'd do & when & kept running back to check what we were doing next! 
No idea about the eating thing, sorry! Calf refuses to eat mushrooms, eggs & potatoes. He can identify the tiniest scrap of mushroom & will inform me "there's something I don't like here"!
He's been fine with chips until tonight - I think he's sussed what they're made of. Might try him with French fries, I'm fairly sure it's a texture thing. Runt, on the other hand, will eat absolutely anything - also including soil, crayons, etc!
Do you think seeing a dietician would help at all? They might be able to make sure he has a balanced diet & work with you on introducing other things. Is it a texture thing or does he just refuse-on-sight?