I have been asked if I would be willing to do a job share at my old place of employment. I would love to return but I have a few concerns about the job share. (a) the hours of the job share would be less than I am doing now and, although I would accept them just to go back, I want to do more hours in the future (I am confident there will be more hours due to ladies on maternity leave at the moment). Would I be prevented from doing more hours if I am in a job share? I know if she left I would be expected to go full time (no problem as I was planning in increasing hours anyway now my children are older). (b) would I be expected to cover the other employee's sickness/holidays?
No one here will be able to answer those queries, you will need to ask your employer.
However in respect of your first query it would be very unusual for an employee to have a guarantee that any request they may make in the future to increase their hours will be agreed. You can ask them whether they think they might be likely to refuse a request because of your job share though. Similarly in regard to the other employee's sickness holidays, you need to ask them. Stuff like that should be in your contract/job share agreement, have you not been given one?