Having just paid for a third new boiler in different properties I am ,as a landlord, really annoyed at the extra expenditure, but I know without doubt that it was my responsibility.
Besides the fact it's common sense it's also the law -Res.tenancies Act part 2 "landlords must carry out repairs,subject to tenant liability for damage beyond normal wear and tear".
For more full wording you can google the Act, download it and stick it in letting agents face telling him that you will withold payment unless fixed or repaired.-or you will have it repaired/replaced and deduct the cost from the rent.
Repeat everything in polite writing.
However, it'S probably best to have another -gentle- word with him first.
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Having said all that, is it a question of the boiler itself -or just a pressure problem that may be solved very easily and cheaply ?? (free if you check it yourselF)