We have a remote thermostat for our alpha boiler, last week the room temp went mental. It now reads 27 degrees c plus!
it is a Sime 8092223 Digital Rf Controller Alpha 6

Any ideas what could be wrong? How it could be fixed?

If a new battery doesn’t fix it, then just swap it for a new one, £90 on ebay for the same model, or £30 for a Salus, but that will involve a minor rewire. The Plumb Center own brand one is pretty good too, it’s a rebranded old model Honeywell.

Second plumbcentre own brand of you need a replacement.