Hi Jonathan. I am no expert but have donee it once with an Atalanta and many times with a Broads yacht. With the fixed supports you need the boat to float off upwards, not try and slide her backwards on the trailer supports.  Which means getting the trailer deep and like Chris says you are unlikely to do that with the trailer connected to the car tow hitch.

Ideally a day with no cross wind and no swell, to avoid bumping the boat on corners of supports and so on.
I think that Richard Hall launches this way – his contact details in the year book.

Out of interest – where are you launching? The start of another family summer afloat?
Cheers, Nick