OK, so as promised, am reporting back....
Train left Banbury, was served a coffee & a packet of biscuits by the onboard host. One of my son's got some warm diet coke. They had no ice. Rather him than me! Crew change at Birmingham New Street then nothing. No Standard class catering (I walked through - maybe it was in the back half), no ticket checks, no sign of any staff at all. Then about 30 mins out of Manchester someone comes along collecting the rubbish. I make a comment about presuming there's no catering (as I was on 4 tickets this could get interesting...), she thought I was looking for the Lobster Thermidor and filet steak, I suggested even just a packet of crisps might not go amiss by that point, to which she scuttled off to the back (actually the front) and after about 10 mins (seriously, this wasn't quick) came back with some snacks and a bottle of water I had also asked for. It was ice cold, suggesting that a fresh trolley had been loaded in Birmingham.
Now, I fail to believe that the job description of the first class host on the 90 minute run from New Street to Piccadilly involves hiding for as long as possible and not proactively offering catering. That would be an interesting way of reviewing the service.
Anyway, had I paid the asked for fare for Banbury-Manchester I'd be properly annoyed. The split ticket represented better value (i.e. about the same as a taxi but without the grief of the M6).
Thanks all!