SHIFT ALLOWANCE???

[nationwide]

I understand that in the new agreement we would be losing the shift allowance of around £12 a week.
I thought I was getting paid this with the supplements you get each month , but I was wrong it does get paid every week.So I haven't been paid it since the year I started,the last payment being November 2005 according to the records in HR.The person I spoke to informed me that when I was paid it it was as a Adoc Shift Allowance NOT an Assigned allowance.There where also several payments of said shift allowance missing within 2005.
As others ,some of which started the job after me are still getting it and I am not I feel a bit aggrieved with the situation.
I have been told to have a word with my line manager about this issue to see why my payments stopped even though as I have said others payments of the allowance have continued.
I wonder how much back pay I would be entitled to???

[the one]

back pay, i got something similar about 3 years back,