How much for a full central heating/hot water clean out?

chubsta

Had my gas boiler serviced by a local firm a couple of days ago (floor mounted with hot water tank downstairs and cold water tank in the loft) - the engineer stated he felt that despite it being 20 years old it seemed to be working well and as efficient as could be expected. There has been a fair bit of gurgling going on recently when I put the hot water on, and the radiators have needed bleeding reasonably regularly - he said this is due to air in the system, he did a quick bleed of the hot water system and it is noticeably better.

As I want to make everything work as well as possible I am considering getting the whole system flushed and cleaned out but have absolutely no idea how much this would cost - it is a chalet bungalow, 10 radiators over 2 floors, boiler and hot water tank on the ground floor, we are in the very South East.

Any advice appreciated