We are have a trunk mould growth issue within our house would you recemend a grill rescue system? ...

How exactly does a tank-less hot marine boiler...
Answers:    Could be damp / condensation etc. Heat repossession system is expensive, why do you think of this odds?
Have you tried turning the heat up? Did the mould growth stop? Heat recouping system saves some of the fry that would be otherwise lost, does not stop damp. Give more details, the more info you can impart in your question the more relevant the answers will become.
What would surface if you use a 4500watt...
sounds like it could be damp or dry rot,does it smell foisty like mushrooms? consult a reputable preservation company for survey. i.e. Timberwise u.k. Rentokill or Peter Cox.