My main queries are: do I have to take annual leave prior to mat leave?
Your employer can ask you to do this. (Unless their policy says otherwise.)
Others have said about taking it after mat leave - if I do this does it automatically get added to end so I go back to work later?
No, it wouldn’t be automatic. You can ask your employer if you can take it after your mat leave but they don’t have to say yes.
The breakdown weekly payments for mat pay seem really low. I'm concerned that by the end of this year I will not be able to afford to pay mortgage etc because I'll be getting no mat pay.
Did you not look into this before getting pregnant?
Someone mentioned to me that there is another form of mat pay that can be claimed but I have no idea how or who to contact regarding this?
Maternity allowance is less money than SMP and isn’t available to people who are eligible for SMP.
Or do I need to go back to work after 20 weeks off when the payment from work reduces?
That’s your personal decision.