I have 2 doors that have a deposit already paid (on 27/4/2010!)

I have 2 doors that have a deposit already paid (on 27/4/2010!) that im waiting on being once and for all delivered and installed by DOORS PLUS osbourne Park.
We have had ongoing problems with them being faulty, they have been back and forth from our house to the showroom multiple times for repairs and the last time Doors Plus damaged them in transit. Hence they went back to the showroom again for more repairs.
The balance on them is still outstanding since these "brand new" doors have never been up to standard and payment was to be made on receipt of doors of an acceptable standard.
Doors Plus currently have our doors and were supposed to deliver them yesterday at 2.30pm (6/4/11), an appointment, like so many others over the months, which they again failed to honour. They today insist we pay the full balance by this sunday, 10/4/11 else they will blacklist us. They are enforcing this prior to them being safely delivered and installed. This comes from 'Charlie' their National Manager! Are they allowed to do this by law?
They have asked us to go and check the doors for ourselves but we have been unable to find time to make a special trip into the city so far to see if they are up to scratch. They have rescheduled next wednesday for delivery obviously only if we make the payment. The doors could well be deliberately damaged by them by their delivery date for all we know and there wouldnt be a thing we could do once they are paid in full.
Our complaints with them are too numerous to mention, with exceptionally poor customer service. Hardly the Doors Plus "no fuss" sevice on their logo.
Please advise me what i should do.
Many thanks, Nancy