You can claim back the difference between HTD/GYH payments received and the HMRC rate, provided your assignment was for 2 years or less. Even if your assignment is subsequently extended, you can still claim. The key principle is that you expected, on the basis of the original assignment, that you would only be working at a particular location for 2 years: so you didn't up sticks and move house (say). Check your JPA assignment order before you claim.