refund for lack of hot water?

Hello ,I have two lots of different guests staying in my two apartments at the moment . They arrived on Sunday night and leave on Friday. Before they arrived I warned them there would be no hot water or heating ,for the first night,  though I did supply them with three radiators for each apartment . The boiler had stopped and I was told it had run out of fuel . Id ordered more fuel and it was going to arrive the day after the guest arrived . They were happy to come so long as the problem was fixed by the Monday. The fuel arrived but then the boiler wouldnt work so my poor guests had another night without hot water and possibly minimal heating , though they didnt say they were cold .

I managed to get a heating engineer to mend the boiler the next day so all in all the guests had two nights out of five with those conditions . 

I said I would compensate them but really dont know what would be fair. Any suggestions or advice would be gratefully received .

Many thanks