Thanks to the help received from CJL and others I took the rib up the M3 from Southampton to Puntney for a day on the Thames on Easter Sunday.
Just thought I would pass on the benefit of my experiences.
Firstly if you have not done this trip you must do it. It was great.
We launched at really low tide from a road called Embankment in Putney SW15. It was suprisingly quiet and plenty of room to park (for free because it was Sunday). Get to Embankment from the B306 Richmond road via Festing Road. I went in by a pub called the Dukes Head and with cars on both sides it was a real squeeze to get through. http://www.streetmap.co.uk/newmap.sr...=newsearch.srf
Its a huge area to launch from, steep concrete to start and then gravel. But at high tide (when we came back) the water was to the very top lapping on to the road. So we went to a sideways on slipway at the down stream end of that area (by the Dukes Head turn in) Easiest recovery I ever made. You could pull up along side and jump off in the dry. Tie up by the wall or the piles, fetch the car and walk the boat on to the trailer. Only available at high tide though, competely dry at low tide.
About 40 mins of steady crusing and we were next to Big Ben. We went under tower bridge and on a bit and on the left is Limehouse Marina. Called them up on CH80. Bloke could not have been nicer. Through the lock, moored up for free (don't know why it was free, I would have expected to pay). Walked down to a Pub called Booty's Riverside Bar. Tiny pub but a lovely sunday roast.
It was much choppier out there than I expected, in the limehouse area especially - waves like a F5 at times. Down by the London Eye it was like a F3 / F4 but up by Chelsea flat as a mill pond.
Hope this helps anyone thinking of making the trip.