Thanks for your reply; I hadn't thought about the possibility of reflux so thanks for the link to the info.
I think I typed my first note about 2 hours too soon; since I posted it, my DS managed 2 x 1.5 hr naps yesterday and a 45 min catnap before bed in the evening. Today he's gone a full 3 hours between feeds & has also been sleeping for all he's worth (1 x 1 hr, 1 x 2 hr 20 min so far and then went down again half an hour ago). He's also eating much better and has hardly been sick so far & generally seems much happier. All I've actively done to achieve this is to shorten the A time by about 15 mins and to try to get him to bed as soon as the 1st yawn happens rather than waiting for 2nd or 3rd one as I was doing before. I don't know if this will last but I'm enjoying it while it does!
Thanks again for your advice; I'll keep it mind if things start going awry again.