So, good news and bad!
Despite a huge amount of Tiger Seal, the rubber still gets lited out of place on the upper horizontal bit of the goal post.
However! This does not appear to affect its ability to keep water out!
The original leak (on the left hand side) has gone and my spare wheel well no longer fills with water.
There is a 'new' leak where the lower tailgate meets the new seal, but I think I have resolved this with more Tiger Sealant pumped into and around the metalwork.
The upper tailgate was an absolute nightmare to close, and it took an hour of finnigling to adjust the strikers to the point where they both lock. I finally got them both to lock into place, but they need a full-on CPR like chest compression on each corner to achieve this...as Mrs C is several kilos lighter than me, I'm not going to recommend she tries this.
I'll keep an eye on the upper seal. Worse case, I'll trim it out, clean off all the black gunk, and re-insert the upper strip of the original seal (which is considerably flatter than the new one, wasn't leaking, and might have a sufficiently low profile to not get scooped up by the tailgate as it opens.)
The lower tailgate is still out of alignment. One day I'll strip off the rubber matting and have a look at what is causing this!