DS and I spent a truly horrendous hour in a big Sports Direct store this morning, after I had sweetened DS's countenance by feeding him at McDonalds.
The staff in there are great (not). Found the youths sizes, picked out 5 pairs that looked possible, asked a member of staff (apparently dept manager) to get them for us in size 4. He scowled at us and said we could only pick 3 to try. I explained that DS has difficult feet, and we may need to try to lots to find something suitable, and smiled at him. He scowled at us and said we could only pick 3 to try 
. However, because we have no alternative, this HAS to be done, we chose 3 styles. They only had 1 in a size 4, so we tried that (brought to us by a surly youth who would obviously rather be somewhere else), but no good.
I left DS sitting on the bench and went off to search the shelves again, came back with armfuls of possibilities (about 7 different shoes), plus two actual pairs in boxes that had been left piled up by the stockroom door, that were size 5s.
After lots of smiling sweetly at the surly youth while we asked him to fetch each style out in 4s and 5s, we have finally found 2 styles that are actually wearable in size 5 - I can't say they fit, but they will do the job.
First pair were airwalk, lovely, wide fitting, but rather garish for school trainers, without proper trainers-style support. I said we would have them. I did wonder if PE teacher would approve, but thought to myself "if he doesn't, I'll give him £30 and DS, and tell him to go and get something better in the circumstances".
Thru perseverance we also found a pair of proper Reebok trainers that work too. The laces open further down than other styles, so they can cope with the width better. So they will be the PE trainers, and the airwalk shoes will be DS's casual shoes.
Thanks very much indeed Ponders and Olympic for your help. Next time DS needs shoes, I'm sending DH with him !!