Do remember though, that by supplying materials your contractor will also be taking responsibility for the quality of those materials, will put right any problems if the order turns up wrong, and will have to put right any issues arising from materials failing further down the line. All at his own expense.
It's not really 'at his own expense' if he would charge a disproportionate markup! That's the whole point of this post...
And also they'll use your time to source and replace.
This is why 60% discounts exist with Jewsons et al. It *IS* a con and it's designed to keep the customer in the dark.
Non-transparent business practices ALWAYS result in somebody getting ripped off.
Establish rules on materials first, discuss 'open book' pricing. Allow them 10% on best price and insist on seeing invoices if you have doubt - but do listen if they have a reason not to use the cheapest supplier.