I would expect SF to always be the cheaper, but there's few inexplicable di fferences I've come across on bulk items.
For instance Ronseal Diamond Hard floor varnish:
OK a 2.5 litre can is £42.98 at B&Q - but if you don't know about the 5 l itre price at B&Q, you end up paying more in the long run at SF.
Actually that 5l can price is the cheapest I can find anywhere.
Same happened when I was buying boxes of MK White double sockets - the box of 5 significantly cheaper at B&Q. I think I also found that some of the ot her MK fittings were slightly cheaper at B&Q (5A round-pin lighting sockets IIRC)
And another was Ragni trowels (the good hardened steel stuff).
Any others you know of where the DIY sheds beat the trade counters?