There's an association of them (which you will find easily enough with Google - sorry but I don't have the link), and there's one I used a year or so ago on Bermodnsey St. Sorry but I can't remember the name of the association or of the law firm, but the person I saw was called (I think) Charles Landau, and (if I've got the name correct) the firm (which is in one of those yards on the RHS if going up B St, almost opposite the Bakery, Ticino's) is made up of three or four names including his. IIRC, there are two or three employment lawyers in this area. The one in B St was nearest to me.
He was pretty good with me, was an employment law specialist, and I was happy with his work. I've never had to use an emplyment lawyer before or since, so I had nothing to compare him to, but as I say I was happy with the service I got. No other connection. Hope that helps.
Did you sign anything before you took the training course that said that you agreed to pay back any fees? That's fairly standard practice, but if it hasn't been explained to you in writing that the cost would be deductible if you resigned, then it may be an unlawful deduction. It may also be in your contract, so you could check that as well.
I did sign a contract when I first joined the company which said training fees would be reclaimed if I resigned within 12 months of the course, but it's a little bit more complicated than that. The course I'm being charged for is the one on my appraisal as being an objective I should aim for. I think achieving an objective as set by your company should not then be chargeable to the employee!
To be honest, the fact that it was an objective isn't really the point. Most employers will only send you on a course because it has benefit for the company as well as the employee. Unfortunately, the fact that your contract states that you will repay fees for training courses means that the employer is within their right to ask for the money back. How long ago was the course? You might be able to negotiate a reduction if it was over 6 months ago or something.
I would be careful with ACAS - I have found in my experience that sometimes they can give incorrect advice, although I have only used them from an employer's point of view. Also, they won't have a copy of your contract to check the clause.
Also, if your employer doesn't recognise a Union, then there's little point in getting one involved, as the employer wouldn't 'back off' when a Union has no right to be involved.
It might be worth speaking to your manager or HR to see whether there's any scope for negotiation over the fees.
I agree with Chuckalata. As a lawyer (but not an employment one, but one who knows employment lawyers!) I would say be careful of mediation through ACAS. Mediation can be very good, but sometimes it merely brushes over major issues to try and reach a swift solution and no one comes out feeling satisfied.