This seems like a rather stupid question but is the Halfords scheme limited to Halfords standard stock? It seems obvious that it would be but I thought that I read in the past that you can order any make through them for the scheme. I suppose it may have been something that was done but no more.

Not limited to standard halfords stock. There are however limitations on what Halfords can get hold of. So no On One, Cotic etc. Halfords also stick very cleanly to RRP and the £1000 limit. So check you can get what you want before ordering.