troublesome tenants

Rent Arrears

I have tenants who are been having trouble with their oil fired boiler and have withheld £500 of their rent however at all times the engineer has been in contact and I have been to the property to attempt to restart the boiler, they have never been without hot water or heating for more than 24 hours and engineers have visited the site within 48 hours and the problem has always been resolved. They are claiming that the house is cold and they are unhappy however there are 2 wood burning stoves in the property more than enough to heat the property and while all attempts have been made to deal with the issues this doesn't seem to be enough. They also offer restricted access and whenever there is a problem they threaten to run off and live in a hotel until the problems are rectified.
Please help, are they legally entitled to withhold rent? The house at no time has been un inhabitable