I had a similar experience.
The agency I was working for had a training school and offered to put me through my class 2. I took their offer and I had to pay half of the training costs back over 10 weeks and they would cover the rest provided I worked for them for 1 year.
I had been working at 3663 through them for the best part of a year on 7.5 tonne work. As soon as I passed my test I was back in there. They started paying me different rates each week ranging from £5.50 to £6.50 an hour (their rate for class 2 is £6.50). Some weeks I got overtime after 40 hours, other weeks 50 hours. They would not give me a copy of my contract (which I am entitled to). When I brought these issues up I just got excuses from them. In the end I left.
A few months later they took me to court for the other half of the money. At court the dippy idiot from my agency office (the owner never showed his face) didn’t even know how much I owed them! Waste of time, another date was set. This time they used a solicitor and I lost because I didn’t submit a letter to their solicitor that they had sent to me.
Beware, if you represent yourself, submit EVERYTHING that you wish to use as evidence. I had a strong case, letters from the boss saying that I should have been paid x-amount blah, blah, apologising etc.
This is how murderers beat the rap!