It is likely that your new role, when you started it seven months ago, would be seen as a variation to your contract, even if you have nothing in writing. As you did this for seven months, it would be difficult for you to suggest that you are entitled to return to the job you were doing before you left on maternity leave.
As you have now resigned (I presume giving them a period of notice) it sounds like they have told you to remain at home rather than work out your notice, which is common. I do not understand why they have said that you can appeal, but if you feel that you have been forced to resign because of the new role then you could appeal. However, I do think that it would be difficult to establish at a later date in the employment tribunal that you have been constructively unfairly dismissed. I am more than happy for you to telephone me on 01793 527141 if you need further advice.
Answer by Helen Climance
This gives a general overview only, based on the information available at the time of writing. It cannot be relied upon in any particular case. Specific legal advice must always be considered to include consideration as to whether the legal position contained has changed since publication. For further information and advice, please contact Lemon&Co Solicitors on 01793 527141 or visit their website: www.lemon-co.co.uk.