I contacted Lewmar in Havant (023 92471841). It appears that the self-tailing Lewmar 30 fitted to my Seal has a single-piece top which should unscrew.
Recommendation is to insert a wooden lever in the self-tailer and to unscrew the head, possibly with the aid of a hammer. I understand there's some risk that the self-tailer will fracture in the process :-(
The chap from Lewmar was very helpful and says he'll email me the PDF of the schematic of this winch and also put it on-line.
Thanks to all,
Richard
Minimosh 28/66