Digistat +2 RF thermostat dead ?

Our Digistat +2 RF thermostat is dead.

A well-meaning friend who was flat-sitting put new batteries in back-to-front while we were away, so that couldn't have helped. We've tried new batteries (right way round), contacts are clean, but nothing.

Also, the unit is now quite old (2009). The battery icon on the Thermostat display flashed intermittently when box was still working three weeks ago, even when fresh batteries were put in.

Before I buy a new thermostat, are there any possible fixes? For example, I tried to replace the 2032 button battery inside in case it had had it, but it doesn't seem to want to be prized out. Does it come out?