Blimey, you dragged this one up from the depths!!! (check the OP date!)
I looked into this last year: http://rib.net/forum/showthread.php?t=28857
As far as I can make out, if the trailer ULW is under 1020kg then it's classed as a 'small' trailer, and NOT counted for the purposes of calculating the GTW. This means that unless the tow vehicle is over 3500kg, then you DON'T need a tacho/op licence for towing (most) boat trailers.
Also, dual purpose vehicles (i.e. 4 wheel drives with at least 4 seats and under 2040kg ULW) are exempt from the op licence regs.