Welcome vwmummy - I haven't btdt (my DS's school has been excellent enough with differentiation that that's been the better option for us) but I'm replying partly to bump your post to make sure that people who have will see it. Many people here have done things like that.
From what I remember people saying about it, the kind of arrangement your DS has can work brilliantly. Things to watch out for are timetabling (danger that he goes up for advanced X while his home class does Y, so that he misses out on Y and later has to sit through basic X anyway) and what happens at the top of the school, when the appropriate class in his accelerated subjects is on another site. That said, the mantra here is mostly to do what's appropriate now and not worry too much about what will be appropriate in future, since it's so hard to predict anyway.
