Apologies for continuing this debate. This year I changed the blocks for ball bearing ones. (The originals were Barton size 3 and the same size ball bearing blocks were straight replacements, apart from the turning block on the deck which required a bit of filing to get the screw holes to line up.)
Unfortunately, it made little difference, I still need the winch.
However, this may be the fault of the forward roller at the bottom of the keel box. Before launching, I noticed that the screws on the fitting were coming loose. I took it out and then found that the plastic roller was not turning and a flat had formed where the keel slid over it. I freed the roller but it may still not be turning because of the flat. I'll replace it over winter. Or perhaps I'm just not as strong as Geoff.
Incidentally, Parkers advised that to remove the fitting, the keel must be jacked up above the bottom of the boat, which allows the roller fitting to be removed by sliding it backwards.