Despite many, many attempts with Captain Tolley I had no joy. I tried a recommendation on a marine forum; https://www.flytesofancy.co.uk/chickenhouses/Burgess-Marine-Creepy-Crackseal--100ml--CCSEAL.html. I had much better results with it, it stemmed the tide, for a while, but it still came back. So, time to step things up. I was going to drop the whole roof, strip and seal the joints but it would need two people to safely drop it so given the current rules (and not trusting the missus) that's a non-starter at the moment. I did decide this morning to remove the headlining at have a look. An hour later:
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I did have to fold the headlining a bit to get it out but what the hell. Sealed everything I can around the joints. Going to let it cure for a couple of hours before I put it back together. In the meantime, will give the headlining a good clean.