Karen
I agree with fellow posters about getting a solicitor to help sort things out. Also, keep communicating with them stating that you are doing your best to sort things out. We were pestered to complete last August and eventually signed on the dotted line in March.........!!!!!
Although we did have valid excuses I found that PW understood that we were doing our best to resolve. Like someone else said, it's in PW's interest to allow you some time but at the same time, they will end up wanting their money eventually and I do think that if you don't have a genuine reason, they are more pushey.
Good luck
Phil