There is more to your question than you actually say. Is the BG man saying you need all your pipes in the flat replacing or just the central heating pipes? Is this the solution to the boiler dropping pressure and just 24 hours of heat? Does "above ground level" mean - first floor or above or does it mean just the pipes you can see?
Just by reading a few questions on here I am very suspicious of BG's attitude to their customers. Each time BG is mentioned it's nearly always connected with a very expensive and often unnecessary quote.
Here's one...
http://www.howtomendit.com/answers.php?id=201391
And another...
http://www.howtomendit.com/answers.php?id=174927
My advice is to find a reliable boiler engineer and end your relationship with BG... Good luck...
Peccavi
January 2009