They can require you do to pretty much what they want (within job description) in the hours that they pay you for. They can't require you to do more hours unless its in the contract and they certainly can't require you to do more hours for no more money. Also some jobs can be doubled up on without much difficulty....eg manning phones that don't ring constantly, checking people in for two clinics instead of one....and some jobs are much more difficult to double up on eg typing up notes and letters. If you believe that you can't take on more work in the hours that you do then you need to sit down with your manager with a list of all the jobs you are being asked to do and find out what you can drop to fit the extra work in.