18 months ago a colleague investigated this and went to International Towbars, 3 Castleton Close, Leeds, West Yorkshire LS12 2DS 0113 234 0076 (details from google) She was very happy with the job and the price.
I'd ring a few as it depends what car, what type of tow-bar and electrics etc. I found that the cheapest for My old Nissan wasn't when I switched to the Mitsubishi.