Hi W R,
Thank you for your question.
The boiler would be a revenue deductions which would be allowable against rental income.
We would class the connection of the property to mains gas as not allowable against rental income as it comes under capital expenses at PIM3010. This states that at bullet point one:
"The expenses on the gas connection would be allowable as an expenses if the property was ever sold".
PIM3010 - Capital allowances
Thank you