If, as you allege, no tenancy agreement is in existence then no rent is legally chargeable on the property. Simply advise your daughter and her boyfriend to cease making any such payments with immediate effect. Make it non-negotiable.
But a tenancy agreement will not rectify the faults to the boiler; for this they will need to put in writing a list of their complaints and forward them to the landlord, giving him/her a specified period of notice in which to effect repairs. An examination of the boiler by either British Gas or any Corgi registered installer will help towards listing the faults (if any), but there will be a charge for this.
If the landlord still refuses to effect any repairs which may be found necessary, and no tenancy agreement has been drawn up, and your daughter and her boyfriend are not paying rent, then the landlord will have to apply for a court order to evict your daughter and her boyfriend from the property and it is there where the landlord will meet with resistance. No court in the land has the power to order an eviction from a dwellinghouse in the absence of a tenancy agreement.