problem with boiler / british gas homecaqre

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british gas have come out to repair the boiler many times in the past month (4 times in the past week) each time they say that they have fixed it but the tenants phone me the within a day or so to tell me they still have no heating or hot water.
I have now agreed to replace the boiler but I am getting a lot of hassle from the tenants who tell me that I have a duty to supply them with heating/hot water and that they expect to be to be compensated for the inconvenience.
I have told the tenants that I have done my best to maintain the supply of hot water/heating to the property by having the homecare agreement with british gas and that british gas have not written to me saying that they cannot fix the boiler or that it needs replacing.

I would be grateful for any advice please
a) How much compensation should I give them?
b) Information on regulations on boilers we are allowed to install
(combi boilers only)
c) any reasonable corgi regestered plumber in London (E15)