System is Potterton Suprima, 215 ltr Megaflow, serving 11 radiators (all on TRV) and 3 towel rails, (no TRV), no room stat. 2 way valve for CH was replaced by friendly plumber a couple of weeks ago as it expired in a shower of sparks for some reason. Problem is: yesterday morning and evening, boiler fired up as normal and within 30 minutes all rads were toasty hot as usual. This morning, however, boiler fired but after 1 1/2 hours the rads and rails were barely warm to the touch. DHW is fine. Any ideas?? Intermittant pump?? I'm not a plumber so be gentle with me please!