Hi oz-plumber I have ideal coming tomorrow to look at it. It only seems to be when the cold water pipe feeding the boiler DHW in and feed to bathroom gets a banging noise in the pipe. I was thinking of fitting a check valve it never happened with the Worcester Greenstar boiler. I will see what ideal says tomorrow then see.Being honest, there are 10 or so things that could cause this.
Try dropping the DHW temperature down to 50C or even 45C and see if that helps the issue.
Let us know.
If the boiler is only 4 months old, might be worth calling Ideal
Reply to the thread, titled "Combi boiler DHW lukewarm" which is posted in UK Plumbers Forums on Electricians Forums.