Mothering Sunday is on 30 March, so you have over two weeks to go. However, restaurants and pubs do get booked up very early for this day so you should lose now time in doing so.
If you find that your local restaurants are already booked, it may be a good idea to try online. If you put 'mothering sunday restaurant birmingham' (or whatever your nearest large town or city is) in a search engine, you should find several offers which you can then check out. If none of these work then your best bet is probably last minute.com - this site is updated regularly so anything on there is still available. They suggest gifts and other treats such as spa days as well as meals out, so if you can't get a restaurant you may get an idea for a different treat for your mum instead.
I hope you find somewhere good.