I use EH Smith for pretty much everything outside of the consumables/paint/sealant etc which I get from Screwfix. I just can't be hooped with running/ringing around to save a couple of quid, and I don't have the patience for dealing with the people mooching about in the DIY sheds like B&Q, Selco etc.
I ran the numbers year before last on material costs over a few jobs and there wasn't much in it. A bit like supermarkets you'd get timber cheap at one, but cement was dear there and cheap where the timber was dear.
The one thing I could see was the cost of delivery charges was quite high for some stuff, particularly single packs. So I bought a cheap Iveco tipper which makes a very modest saving on the year comparing running costs to a years delivery charges. However it makes a massive difference in knowing exactly when stuff arrives and where it's placed.
The long and short of it is that direct material cost is irrelevant as it's a direct charge to the customer. So for me it's more about making things easier than cheaper. Obviously the work I do and range of materials I buy will be quite different to working as a plasterer, but I reckon the principles are the same.