People believe that Builders merchants are for tradesmen only, this is not true. Get to know the staff in your local builders merchants if you are a regular DIY'er and you will get a much better deal than at a DIY store.
I have to agree. I've worked in 2 of the national merchants and one independant over the last 9 years, you will, 9.5 times out of 10 get better advice from a merchant, than you will at a shed. The reason:
Merchant staff are in contact with builders and tradesmen on a day to day basis, so they need to know things that tradesmen know. If tradesmen go to sheds (B&Q etc) then they pick up what they need, pay and go. There is no interaction between customer and seller. Added to this merchants will generally send their staff on training courses (I myself have been on many courses, covering bricks, blocks, timber, plasterboard drainage etc etc).
There is also the lost art of negotiation, something else you don't get at the sheds.
Whilst I don't profess to know everything, and I most certainly am not a tradesmen, I do know my stuff (well.. sort of ;) )
Has to be the builders merchants, I get better advice ast they know their products. Only go to B&Q for one off items when they are shut. I find plumbing merchants (City Plumbing)and electrical wholesalers very good for advice and value for money (TLC and BEW).