Definitely this. Work out what the landlord owes you and deduct it from your rent to pay in the next 3 months - this is far easier than trying to get money back. And don't fanny about with only asking for 50% either; it's the landlord's responsibility if building work needs doing or boilers need...
You paid for new boiler and other repairs in a rented house??! That sort of thing is most definitely the landlord's responsibility, and there should be some money for you to claim back off of them.