I know I've been a fool but am I being ripped off?

I'm doing a total renovation and remodel of a 1950s bungalow. I received a good quote from a roofer recommended by my hairdresser so decided to replace the whole roof as well as roofing the new extension. He did not provide a written quote and asked for payment for the tiles straight away (back in August and they're still not on the roof). He appeared very friendly and supportive but as time has gone on, the costs have escalated for different things that he says I need and he has always wanted the money up front, including the labour costs. I have now paid him three times the original quote and three months on all that's been done is the roof stripped, felt and batterned and the facia finally finished today. Is it normal to ask for payment in advance like this? He has become quite defensive and refused to fit the velux windows because I did not want to pay any more labour costs as I felt I was well in credit with all the advance payments that I'd made. I feel he's over chancing me for things e.g. £5,800 for the lead valleys and flashing but don't know how to find out for sure and then how to tackle it when I do. Any advice would be very gratefully received.