Finally finally finally ……..got rid of the old silicon. Could not get a flat blade onto it because the silicon lay in a gutter finger thickness. With much general gouging and scraping I got rid of 90% of the volume. Using plenty of acetone still did not seem too effective, however in retrospect it might have been softening it a bit for more scraping. (Porters recommended acetone for cleaning before fibre glass jobs when I called them a couple of years ago).
Looking for silicon sealant remover in DIY stores…….B&Q no joy, nor Wickes….finally found some in a marine chandlers although the packaging makes no reference to boat uses for the product……..made by “GEOCEL”…..(a strategic partner of Dow Corning it says on the box….interestingly it was past its use by date and was their last one so whether it is still made I don’t know. Cost £3.50. Anyhow its a gel that you leave on for two hours. This did the trick but still required more scraping…….this time with finger nails (so it was not a miracle cure). Finger nails now suspiciously clean but so is boat where silicon was………probably could have glued hatch seal over the traces of old silicon anyhow but proper job! Thanks for your help David and Peter.