Digistat RF1 Wireless System?

My Digistat RF1 is on my kitchen worktop, set at 21c. The temp though is a steady 14c! The green light is showing on the receiver and the red flashes now and then. The boiler though seems to come on then switch off before it's done much work. The flame sign is showing on the RF1. I have noticed occasionally that the RF1 has no picture showing, but when I've pressed any button the pic comes back. This may be a stupid question, but might the batteries be causing the problem? I haven't changed them since the system was installed in 2005! In my defence I haven't seen the flashing battery sign either! Please help, I'm freeeeezzzzing! Thanks. Pam