myson orion pump over run stat?

I have a myson orion system boiler ,when the heating has been on and turned off by switch or room stat. The boiler,the primary flow and return bang like hell as there is water hammer in the system ,does anybody know if the boiler has a pump over run stat. If so where could i get one. Or any possible causes for this.Also when the stat is turned up on the boiler its becomes more frequent

cheers