What does it specify in your company maternity policy? They are legally required to have one.
In my own company, once grounded due to pregnancy, crew members receive 90% of their average salary over the preceeding 3 months. This lasts until commencing maternity leave, when they receive the first 6 weeks at 90% of their grounded pay and then statutory maternity pay until this is exhausted.
If a crew member were to fall pregnant again whilst on maternity leave (as in your case), then payment would be 90% of your monthly SMP until your second period of maternity commences fully and you are eligible to claim full SMP once more.
Different companies will have different policies, but in this industry, especially in the UK, I wouldn't imagine many employers will offer more than the law requires them to do.