thanks for quick reply guys. All this has been considered. We are aware about getting permission before doing stuff, but up to now the tenants have been good and I'm trying to avoid getting heavy. I realise the agent, who receives their fee has a vested interest in keeping things sweet...but as they say...taking the legal approach is long and difficult and almost always, the guilty party is given the opportunity to put things right first anyway.
They suggest it's Far better to let the tenants have the opportunity to prove the alterations were done by a qualified guy and for them to get the dodgy wiring sorted out. In principle, we have nothing against them adding this thermostat system..but were ****ed off they didn't ask first and now it seems to be potentially dangerous.
So, back to my question...what can I request from them/him to prove the boiler has been checked...and all is now good