Welcome to Flyertalk AmandaB, welcome in particular to the BA Forum, though it's an unfortunate incident that brings you here. I hope you sort out a decent Plan B and that you keep on posting here. All experiences are welcome, and it's good to see new faces here. Welcome onboard.

You seem to have agreed a settlement with BA, so apart from holding BA to the letter of that settlement, that's probably about it. I would absolutely insist on the Amex voucher being extended, or an alternative provided, and of course you can point out that there is a tape of the agreement that you had. If this is what was agreed, then both sides need to stick to it, and you should hold them to their promise, or renegotiate from the start again.

Good luck and keep us posted.