SSP should be payable for 28 weeks. (Since your employer reclaims it from the tax man, there's no reason for them to avoid paying it). So you should be getting £81.60 per week.
Any additional amount you might have been receiving is outside of SSP and subject to the terms of your contract with your employer. (Many people only get SSP but some employers pay additional money for a certain period of time).
If you were dismissed from your post simply because of a fairly short-term illness you would be able to refer the case to an Employment Tribunal, seeking either reinstatement or compensation because of unfair dismissal.
But if your health declines to a point where you can no longer do your job your employer can lawfully dismiss you.
Chris