Hi Dilly I don't think that's a clear cut one but here are my thoughts.
First thing - they cannot refuse you a promotion on the grounds that you are pg (that would indeed be discriminatory). So them saying that you cannot have the promotion until after you are back from mat leave is questionable, I think.
Second - you are entitled to return to the same job on the same terms after your mat leave. So if it is agreed now (before you go on mat leave) that the new position is part of your terms of employment, you are entitled to return to that position after mat leave. If someone else is doing that job while you are on mat leave they should only be doing it on a cover basis and you are entitled to get it back on your return.
So...
Assuming the new position is something you could start doing now and keep doing until you go on mat leave, I would push for them to agree to this (and to confirm it in writing).
Obviously it doesn't do much for good relations to start throwing around discrimination accusations, so I would stick to something along the lines of "I'm really keen to do this and there's no reason I can't start it now".
Failing this, if they are insistent you can't start the new position now but can have it when you are back from mat leave, your best bet would probably be to get them to put this promise in writing.
HTH. Apologies to everyone else for boring you .