hi bohemianbint
It doesn't matter whether the extra work was specifically outlined in your contract, if that's was the work you were doing most of the time for a significant period of time, then that's your normal job, and similarly the money you were getting is your normal salary.
You are not entitled to return to the exact same job after AML if it is not reasonably practicable for you to do so.
However if your boss has kept the guy on who was doing your job while you were away, that means the job is still there, work is still available so you should be doing it.
When you say you signed a contract saying you would come back on reduced hours but nothing else would change, when was this? Was it prior to you going off on maternity leave, in other words did you arrange with your boss before going off what hours you would want to work when you came back etc?
If as a result of your maternity leave you are now doing a different, lesser job in terms of status and financial reward, but the work you were doing beforehand is there but being done by someone else, you are being discriminated against, so I would say you have a good case.
You do need to get proper advice from someone who can look at your contract, both your original one and the thing you signed about coming back after maternity leave. Persist with the CAB, they shoud be able to refer you to someone who can help you, often solicitors will offer a free consultation, and there are often free legal clinics you can go to as well, but your local CAB will be able to point you in the right direction.
In terms of going to a tribunal, difficult to comment without knowing more, I would not like to comment too much on your chances of winning etc without seeing all the paperwork and hearing all the details. However compensation for sex discrimination is not capped but this explains how it is calculated in the event of a successful claim.
I would put in a grievance in writing, here is the basic statutory grievance procedure to follow if there isn't a more detailed procedure where you work. Basically you have to put it in writing and your boss has to meet with you to discuss it. Doing that will ensure no one could hassle you at a tribunal about not having raised it as an issue.
In the meantime do try and get some proper legal advice giving all the details about your employment and taking all your contracts and paperwork, it is not necessarily as straightforward as some, so it's important someone looks at all this before you put in a claim. You can certainly put in a grievance immediately though.