Hi, just joined here to thank @Gary for his tip on the tow rope.
I bought one but didn't like the weighty metal hooks (~360g together), so went to work with a dremmel and cut them off, replacing them with some lightweight carabiners. The carabiners (£10 from Go-Outdoors) came complete with an extension loop which was just right for fitting over my son's stem as an anchor point. The other end just clips onto a saddle rail. With a little imagination you can just wind the tow rope around the lead bike top/head tube when not in use so it's very quick to hook-up.
Only briefly tested so far, but looking good. Full test in Scotland in a few weeks.
I did like the look of the TowWhee, but this works out half the price and with a better attachment system. It's maybe a little bulkier when not in use.