Settlement Agreement Negotiation Advice England

Hi,

I have been offered a settlement agreement as part of the performance management process. The reason that I've been given is performance below expectation, but that was never communicated to me. I had checked verbally over the past year with my coach on action points and she had said it was due to a lack of experience and we would work on getting that experience. She was not informed of the performance management process, and was in shock when I informed her.

Truth be told, I am still in shock. But, am also tempted to negotiate the settlement agreement. Current offer I've been given is 3 months PILON and 3 months pay as ex-gratia. Is this reasonable or would you negotiate other items?