BG are talking out of their backsides (or trying to con you into some expensive work).
If your pipework is 'clogged' the system can be given a dose of 'chemical flush' to dissolve it. Then the system can be flushed through with clean water before refilling.
As for 'no parts available' for the CSI 85T - copy and paste this link into your browser to prove BG totally wrong.
http://www.boilersparesdirect.co.uk/parts-online/shopdisplayproducts.asp?id=3692&cat=CSI+85+%28CONDENSING+%29
Vic Webber (Plumbline Contractors Ltd.)
March 2008