I was seconded to a Senior position in Nov 2010 but remunerated at my lower level salary for 16 months. During restructuring, my Manager wrote a business case motivating that I should be placed in the new structure due to my critical skill. I was given a (provisional) appointment letter in April 2012 that placed me at Senior level post I was seconded to and my salary was adjusted accordingly. I never received employment letter confirming my appointment as the company was still busy with restructuring. My salary was reduced in Dec 2012, employer says it was a error. My Senior Manager requested me to continue performing the functions at Senior level but remunerated at junior level.

Please advice