Appropriate Discount Pecentage For Tenant

Other

I really need help on this. I am managing a flat and it has been let out for 2 months now starting 1st September. For the 1st month, the central heating didn't work but i was not notified until the second month. The hot water tap in the kitchen was broken but i was aware of that. I couldn't get it fixed till the 2nd month. The tenants informed me of the faulty central heating in the 2nd month and on bringing an engineer in, found out a new boiler had to be fixed. the central heating and hot water tap is now working as of 15th , November.
Pls advice on the appropriate dicount percentage to apply to rent and the start date of the applied discount. The tenants are insisting on a 50% discount from the 1st month which has not been agreed upon by myself.
Hope to hear from you soon.