Thanks for asking. Went to GP again this morning, who wasn't happy so phoned hosp, who also weren't happy and asked us to come in. So we went down there and spent several hours sitting around while people scratched their heads and asked me if I was washing his clothes in acid / feeding him prawns topped with kiwi with peanut chasers, etc etc.
Eventually they pronounced allergic reaction (4 days, constant, not relieved by antihistamines and with no trigger, yep, sounds right to me), told me they wouldn't give him anything stronger unless he started having trouble breathing and we were sent on our way. Oh, the best bit - the doctor said "It could be a reaction to the change in the weather". Honestly, I couldn't decide whether to just weep or hit him. So I just stared at him in miserable disbelief until he looked a bit ashamed and looked away 
He's been miserable this afternoon, won't play, won't let me leave the room, just crying and scratching. But in the last hour I have put him in a tepid bath with loads of bicarb of soda, which he screamed bloody murder through and hasn't helped the rash BUT has stopped him scratching quite so much, then got Manstew to get some Eurax cream on way home and have slathered him in that - he's next to me at the mo naked on our bed, still a giant hive but not scratching. At this stage I've given up trying to find a solution, I just want him comfortable.
Personally I think it's a reaction to a virus. Whether it's the slapped cheek virus or not it seems I'm not going to find out. Will ring midwife in a couple of days and just say I have been exposed to it and ask for a blood test for myself. I had a big cry earlier but feeling better now, mainly cos the Eurax seems to be working and I have the use of my arms back, instead of holding a bawling toddler all day!
Sorry, all very me me me. Sorry. Dead legs to all.