wall mounted fan assisted Sime 102 delux combi gas boiler works ok as far as central heating and water is concerned, but constant overflow from diverter valve and can't release pressure as it just rises again. constantly at 3 bar which i believe is why the diverter valve is operating. where is the pressure coming fromand how do i stop it?
Have you recently had to top your system pressure up using the filling loop? If the answer is yes- perhaps after some DIY or after bleeding some rads, then check you have turned it off properly- then drain some water off the system via a drain cock.
The easiest way to check your filling loop is turned off, if it's an external one, normally a short braided pipe- is to remove the loop downstream of the isolation valve on the mains side. You should not have any water passing the isolation valve- if you do, this is the problem.
If the above isn't the cause, then I'd recommend calling someone out to troubleshoot it as it could be any one of a number of components at fault.