BN - would be worth having a look *caveat not a financial advisor* but iirc in the HMRC pamphlets somewhere is an explanation that if your job is expected to be less than 2 years then you can claim tax relief up until the point at which you find out it will be more than 2 years. Likewise however, the opposite applies so if you get a 3 year posting and then are short toured iirc you get short-changed.
Well worth a look into the details on HMRC's site and if in doubt, give their helpline a call to chat through what you think you have.