Geoff, here's a little wrinkle (don't worry I have lots and am happy to get rid of one. Sorta like plastic surgery)........
Get whats called a 'turning block' (a cheap second hand one will be best) and fix it (bolt or tie it well so it can't slip) to the very back end of the trailer. Then get a length of rope about he length of the boat plus a bit for knots. thread the rope round the pully block so the rope is turned 180deg (turning block see!) and both ends of the rope are about 2/3rds of the way to the front of the boat. Now tie a loop in one end and fix a strong shackle on the other end.
To use....... On arriving at the slip and removing all boat tie downs, if the slip is shallow sloped. Release the winch rope from the boat and undo it until it can be hooked on to the loop in your nice shiny new 'launching tingymejig rope', the shackle end of the rope then needs drawing up (by letting more winch line out) so it can be shackled to the bow eye of the boat.
I bet your there already! Yep , you winch back the winch line you let out and hey presto the boat is pulled BACKWARDS down the trailer to about the 'half way off' mark. Either get your feet wet or do a teetering tightrope act down the trailer and undo the shackle , give the boat a shove and there she is floating away on the tide because you, YOU TWIT forgot to hold on to the bow rope
Mad Mike