Yes - dig them out first. Weedkiller used now, unless its something like basic Weedol, may well sit in the soil which won't be good for new turf. Even if you use weedol, you'd have to wait a couple of weeks and probably treat again and then wait to make sure they're dead.

I just realised you mentioned brambles - no weedkiller will get rid of those anyway, so you need to dig them out. If you can't because they have large woody roots, then drill into the woody root and fill up the holes with SBK and cover - make sure you carry this treatment out a good 3 inches below the top of the soil, and cover carefully. As long as you don't spill the SBK on the soil anywhere, you will still be able to turf, but it is a risk.

if your not in a big hurry you could cut all what you have down and cover your garden in black polythene for a couple of months or so.this will kill everything .no weed killer and unselective.

Thanks for the replies,
Wish I'd never started. Its a shared garden that was overgrown and not used at all so decided to take it on. Everyone thought a new lawn was a great idea but no-one was interested in helping with it. At work last night have decided to go for it and plant the lawn and pray. It can never look as bad as before. I will keep you posted as to whether the grass grows faster than the weeds!!!!

lol your welcome we try and help if we can good luck your probably wright ill keep my fingers crossed xx