Julian, can you recommend a good place to launch and store the car and trailer?
Yes, my three favoured slips are:
Bradwell Marina on the Blackwater - about £20 for in/out plus car/trailer parking. Trailer/car not secure so need a good clamp. HW +-4. Good.
Titchmarsh Marina, Walton Backwaters - they recently charged me £36 for in/out plus parking plus 2 nights berthing in the Marina which I thought was quite good value. Without berthing, <£20. Trailer is always kept behind a locked gate here. Really nice place. HW+-4. You can technically use slip all tides but it gets increasingly muddy.
Woolverstone Marina (River Orwell, Suffolk)- about £36 for in/out plus parking. No secure area, needs clamp. Slipway good for HW+-2.5, but cross currents if not at HW. Really expensive, but probably most accessible - about 10mins from A14 whereas the others involve longer forays into deepest, darkest Essex...
Also a v. Steep slip at Suffolk Yacht Harbour which I don't use since it isn't good for single handed launch/recovery - they have a wire thing there which connects between car and trailer. This keeps the car on the level and hence prevents it from sliding down the slip but means the driver (usually the skipper) can't see what's going on, so need at least one other to wave arms.