Am I entitled back pay for working outside graded above my station?

Hi all,

I work for a large company within the UK.

I’m currently wondering if I can claim pay back for working outside of my pay grade, without knowing.

I was taught a skill at work, this skill requires the higher pay grade. However I was never notified of this.

I completed the work for 5 months before taking a 2 week holiday.

Upon my return I can see that I am no longer skilled in this area and they removed it without telling me.

I waited 2 weeks before asking just to see if they’d tell me. They didn’t, I had to ask.

Upon asking they told me they removed the skill as it is of a higher grade and I shouldn’t be doing it.

My question is, do I have grounds to ask for the difference in pay for the months I completed the work?

The whole situation is very shady and management are reluctant to give me the time of day in voicing my concerns. It goes to my line manager and no further.

Thanks for reading guys