Mik007

Well it is possible to take it out to clean it,but its not always possible to get all the crap out,then youve got to drain it all out again to fit a new one.
Personally i would get a new one,then fit an inline strainer/filter to the return pipe under the boiler,that should minimise any further build up in the new one.

Hi,
It sounds to me like the hot water heat exchanger is blocked,or restricted,very common fault due to system debris.The isar seems to be particularly prone to this.
The clonking and banging noise is the boiler overheating,accompanied by a fluctuating pressure gauge.The boiler will probably run ok on central heating.