Advice needed: Salary offered, then a lower amount confirmed in writing...

hawkis

Long story short-ish (this has been going on since June), here is the sequence of events:
December - Offered a job internally on a trial basis for six months at X salary. Confirmed in writing.
June - Met with line manager, their line manager and another colleague and was offered Y salary which I accepted.
August - Finally received an increase in pay but £1000 less than the offer of Y mentioned above. Also received a letter confirming the amount of £1000 less than Y.
Informed my line manager who confirmed that they recall from the June meeting that the amount offered was indeed Y. They contacted their line manager who now refuses to believe that they offered me Y in the June meeting.
My line manager has repeatedly confirmed that Y was offered in the June meeting. The other colleague also recalls it was more than the amount I am now receiving but not the exact amount, but it was definitely more.

Hopefully the above is clear!
Anyway, what can I do from here?
It's clear that internally the person who had made this decision (and error) is not budging but I and my line manager both recall the meeting and amount offered. They have said they are willing to back me all the way although I would understand if they were to change their mind down the line if I have to take the matter further.
Any advice gratefully received.

agrinnall

I would have thought that you would have to file a formal grievance, which should then be investigated by somebody who is either independent (eg HR) or is at a higher level than both of the managers.