Thank you all. I knew there would be a good body of experience out there.
I've checkled the pay slip and it is GYH(M), not HTD.
Here's the thing, whilst I get posting notices for 3 yrs I am almost always posted inside of 2 yrs (Exec stream - my fault). So, whilst the document says 3 it's almost implicit that the posting will last circa 2 yrs (usually less).