Advice - training at work - wage deduction

I started with my previous job in January and provided the company with my CV and certificates stating that I had a first aid qualification that i recieved only a few months prior.
After a few months into my employment my manager told me she had booked me on a first aid course, I asked why as she knew I already had this qualification and she told me I had to redo it for the company. I was then given a letter to sign stating if I left within so long I would have to pay a percentage of the course. I told her I didn't want to sign it and joked that it wasn't because I planned to leave but just in case. She said I had to sign it in order to do the course and not to worry about it. So I did.. It's hard to explain but she made me feel like I had too and I didn't want to pee off my new boss.

On the first aid course the trainer from the Red Cross asked why I was doing the course again after a short time. I explained my situation and told him I was practically forced to sign this letter. He told me that I shouldn't of been made to do the course again as it was valid for any employment he also said I could not be forced to pay as I already had a certificate.

So, a couple of months later after 6 months of employment I've left. I've recieved my final payslip today and they have deducted £154.40 for training.

So my question is, can i fight for this money back or have a doomed myself for signing a document?

I now have two identical fist aid certificates and I'm £154 down

Thanks guys