Biasi Prisma 24SE...hot water switch resets on cold days!?

I have a Biasi Prisma 24 SE, which was serviced, cleaned and had a replacement diaphram fitted less than 1 year ago. With the onset of the recent colder weather, the little red reset switch pops out every few hours and needs to be pushed in to get any hot water!
The pressure gauge for the heating system reads 1.5 bar (I have recently bled all the rads) and the temperature gauge is at 50 degrees. Surely the diaphram won't have gone again already?!?