Employer deductiong from salary, but not paying into my NEST penson pot

I have been working for an employment agency for a couple of years and around a year or so ago, I received a letter from them telling me that I had been auto enrolled into the pension scheme NEST and I should expect a welcome pack from them shortly.
Sure enough, the pack arrived and I was auto-enrolled.
Now, about a year later I have received a letter from NEST informing me that they have reported my employer to the pensions regulator for late payment of contributions.
I have to be honest, I had never logged into my NEST account online (bad I know), so I logged into NEST and despite my employer deducting small amounts from my salary every week and showing on my payslips as 'AE Pension' my contributions were apparently - zero. Not so much 'late' contributions as no contributions whatsoever.
Naturally, I found this a little alarming, so I called Nest just to be sure. I told them and received their letter and I also asked them to confirm the amount in my pension pot and indeed, my pot was empty.
In the letter it said 'despite repeated warnings' to my employer... so it's not as if there's a simple administrative error or a one-off or anything like that.
My question is, am I within my rights to contact my employer, ask them for an explanation and if not satisfied with their answer - ask them to refund all the money they have deducted from my wages over the last year?
I know there are rules in place about taking money out of a pension fund until you have reached the required age, but the fact is (or appears to be) that they haven't actually paid it in - despite deducting it from my wages.
Trust has been lost here for me and I don't intend to work for this agency again. We're not talking a huge amount of money here, but of course that's not the point.
I plan to find work elsewhere now, and will most likely use a different pension fund so even if the agency in question offer to pay everything they've deducted it in to NEST, I don't want them to in all honesty.
So, can I ask them to repay the cash?