You can charge them a reasonable hourly rate based on the type of work done and the skill required to do it. So cleaning can be charged at £9-15 per hour (depending on the local rate); skilled joinery work at £25 per hour.
It's nothing to do with how much rent you are losing, as advised above.
'Pause you who read this, and think for a moment of the long chain of iron or gold, of thorns or flowers, that would never have bound you, but for the formation fo the first link on one memorable day'. Charles Dickens, Great Expectations