worcester bosch 280 heating rads while running hot water?

While running the hot water Ie.Shower, all the radiators are getting hot. Now I know that during DHW demand the heating is 'switched' off, then when DHw demand stops waste heat is shunted to the rads, but never both at the same time.
This has always been the case until tonight when this has append. Also the pressure rose to 2.5 bars, which I don't think it normally does.

Does anyone have any ideas what may be causing this?

(worcester bosch 280 F)

Pete
June 2011