Thanks very much for all your responses - it's very helpful!
Just to let you know that this plumber was recommended to us and that they have invoiced us almost £950 for this - it's not cash in hand and we didn't intend to have employed a cowboy :( We've learnt a lot from this and will look...
Is it normal for a plumber to leave a leaking stopcock? We had a plumber in to fit a new bathroom suite and they have left without fitting the bath panel (said the bath waste pipe was at the wrong height); has left the toilet at a wonky angle to the wall, not in the middle of the wall and not...