what are you actually asking here?
have you asked your midwife for another mat1b? they may be able to help.
the first 6 weeks maternity pay is based on your previous 3 months pay or something so yes that may well not be enough.
is the boss keeping your mat1b for a reason?
was there a second reason you were in hospital or was it for the SPD, ive never heard it to be so bad that a woman is admitted, was it something else that caused you to be admitted?
have you spoken to the CAB at all?