descale Rayburn boiler?

I have a Rayburn 380L LPG boiler.makes knocking noises when turned on and switches off before circulation takes place.Plumbers says it has blocked heat exchanger.It is 9 years old.Estimate to flush is £350 which seems a lot. Any ideas if I can tackle it myself? I have lots of DIY experience and have done quite a lot of plumbing.