If they go off sick, do you have a return to work policy and how do you manage excessive sick absence?

I think there are a few ways to manage this. ;-) Firstly you can always offer someone the opportunity to take annual leave rather than unpaid emergency leave. Secondly, as Jules says, you can robustly manage sickness absence with return to work interviews and triggers. Thirdly you could, if you wished, introduce a certain number of paid emergency days off.

We allow up to five paid days per year - but they must be for emergencies and they can only be used if someone doesn't have excessive TOIL/annual leave to use up first (defined as having less than two days leave per month left to take up to the end of the leave year).