How can I fix my Danfoss TP5000 + RF programmable room thermostat ?

This is a wireless thermostat as combi boiler is in roofspace. No problems for three years, but now will not come on at alloted time without going into loft to turn on manually. Boiler is working fine, had plumber check it out, he suggested we reset as frequency was lost. Did this according to user instruction book. This worked for a few days, but now its done it again. Does not indicate that new batteries are required so don't think its that. Unfortunately, now out of warranty. Can anyone help ?