Oh that's interesting and a great help to me. I've just phoned to clarify a few points and it is as on this chart with, as you say, no need to be registered elsewhere to get the visitor licence...
With my 340 Zodiac I'm in tariff class 30 (with 4hp) or 33 (15hp) on that chart so regardless of using the 4hp or 15hp it is £8.50 for a day. They say the easiest way is to phone them and pay by card then they'll send an email with the registration permission which you can print out or carry on a smartphone to show if asked. The only nuisance is you have to nominate the day so you can't phone on a Friday then decide on Sat or Sun depending on which looks best for the weather.
If I go for a 7 day licence this is still £8.50 for the 4hp but jumps to £17.24 for the 15hp. Again you have to state the dates the 7 days cover and they have to be consecutive not spaced over several weeks/months so actually the 7 day is no advantage to me.
The daily rate will suit me as I like to give any new to me or recently major serviced outboard or boat outfit a "proving" run on the river (which is only 15mins from our house to the slipway) before going for the near 200ml round trip and full day out to the coast. Also the Mrs likes a drift down to the riverside eatery a few days a year and the daily rate is ideal for this.
It also means now I don't have to keep the 4hp just to reduce the yearly river rate by £200 I can sell that and reduce the garage clutter by one more thing.
So many thanks for checking that out Popeye otherwise I'd have continued to believe what I was told last year.