GETTING A NEW BOILER

EP34

I had a couple of bursts in my kitchen with my heating pipes and the plumber who fixed them for me said that my pipes were of poor quality and needed replacing so as my boiler was over 30 years old it would be best to change to a combi boiler and get new pipes and a new system in. So saved up for a year and then contacted British Gas as I thought they were a reputable company and would do a good job for me even though they were very expensive compared to other companies. I explained to the chap who came out why I was replacing my boiler was due to my pipes needing replacing as the boiler still worked and he said that was fine I would get a new boiler and all the pipes would all be replaced, and that the current pipes would not be compatible with the new boiler anyway. So I agreed to have the work done and it came to almost £7,000, which I paid a deposit for and then am paying the balance over the next three years.
However, they came to do the work last week and have fitted the new boiler but only connected up 3 of the 8 radiators I had before (in the bathroom, hallway and kitchen) and they have said I need to pay a further £2,000 for new pipes as my old pipes are of poor quality and need replacing and are not suitable for the new boiler.
I said that I had already paid £2,000 for pipe installation and connection and that I had said to the chap who came out that the only reason I was buying a new system was to replace my old pipes. But they left me with a connected radiator in the bathroom, kitchen and hallway and said I need to pay a further cost to have the pipes replaced and connected in the front room and bedrooms .
I have looked at my quote and it says £1303.01 for the boiler, £2777.89 for heating system installation, £1903.38 for pipe installation and connection and £1122.24 for tailored extras.
Is this correct, can they suddenly do this halfway into the job even though I had said it was for new pipes? Is this a common occurrence? I do not have the funds to pay this, I will have to add it onto the money that I have already borrowed and I only earn £12,000 a year so I was stretched to the limit beforehand but with these extra costs I do not know how I will manage the payments.
Any advice you can give me please would be appreciated. 
Many thanks

Vitor

Before doing anything else, put this into a formal complaint with British Gas and do not agree to the extra £2,000 until they have explained it in writing.

Ask them to confirm exactly what was included under “heating system installation” and “pipe installation and connection”, and why a job costing nearly £7,000 has left only 3 of your 8 radiators connected.