Thanks for the comments...and so, the story so far...
LO has been eating wheat since we started weaning at six months. He was BF all along as well. I suspect that his symptoms began to show when we from three to two feeds a day when he was about a year old, perhaps the BM was shielding his system somehow.
His symptoms have never been dramatic: no skin irritation, no sleeplessness, no apparent tummy aches, just enormous stinky, sludgy BMs with bits of undigested food in it. Also, he used to poo pretty much like clockwork but then seemed to loose all control of timing, even getting up in the morning with poo in his nappy, which he hadn't done in months. So there was something irritating his gut, definitely. I'm lactose intolerant so we tried cutting out dairy (no difference), reducing his fruit intake (no difference) and now, having fed him wheat so we could do the coeliac test, we've had him on the GF/WF diet for three weeks or so. Poo is back on time! And there's no sign of the big sludgy horribles unless his childminder gets confused and feeds him something she shouldn't.
So he seems to be symptom-free on the GF/WF diet. I suppose I want to confirm it's wheat intolerance and not this 'toddler diarrhea' thing. Perhaps there's nothing wrong at all? I don't want to restrict him needlessly.
And what, in practical terms, is the difference between a GF and a WF diet? I know we need not be as vigilant as though he were coeliac but what can I reintroduce to his diet?