If you had verbal and written offers of full time, then I would say that you have a contract and the school has no right to change it. In your position, I would write pointing out the verbal agreement along the lines of "there must be a mistake". Reserve "kicking up a stink" until it is clear that the school has no intention of keeping to their word. This assumes that you are unwilling to do 0.8. Otherwise, you could agree a compromise with the school that they will allow you to finish your NQT induction in term 4, and will switch you to full time as soon as possible. This does assume that you trust them enough; possibly a bold thing to do given their actions so far. I don't think you are bound to take up the post if you can find a full time post elsewhere. But don't sign the new letter yet.