Only two I've never had problems with, and I've had to rma stuff with both, are
www.overclock.co.uk and
www.kustompcs.co.uk
I've had good service from Dabs as well, but never had to rma anything with them, so I'll reserve judgement.
Just about every other supplier, I've had problems with, especially e-buyer.
Probs with: overclockers.co.uk; scan; Aria; CCL (only minor); Tekheads; e-buyer (really bad); redstore (really bad) and some more I can't think of.
www.chillblast.co.uk are good as well, but pricier than most
In short, online buying does carry a degree of risk, particularly if you can't contact the company by phone or via a trade counter local to you.
But we all buy this way, cos you just can't beat the range of stock and prices.
PS: forgot, haven't had a prob with Komplett either, but, as with Dabs, never had to rma anything.