Thanks to all who have replied to my question - apologies for my late response, but we've had poor signal recently.
We have already been down to Limehouse to talk to the lock-keeper (and scare ourselves looking at the number and speed of the river traffic!) obtained the PLA Users Guide,and been in touch with the SPCC. We hope to join their expedition on May 17th. If we are therefore part of a convoy, do we need our own VHF radio? We don't want to be foolhardy, but it's a big expense, not to mention hassle to buy a radio and do the course just for a few hours on the river if it's not essential.