MonkeyBall
Your offer is an eminently fair one and, I venture to suggest, will undoubtedly be seen to be such in a court of law. The law is essentially about "reasonableness" and it does seem to me that your employer is showing very little of same.
My advice, for what it is worth, is that you should tell your employer to get on with it and take you to court. Point out to them that it will be a waste of their money but will certainly keep their lawyers happy.
Good luck, old bean. Hope your next employer is less of an a******e.