Whenever a problem with sealing/ adhesion comes up, the answer is "Sikaflex".
I needed some to seal a leaking rail so went online and looked for "Sikaflex" thinking that I could get it online ,delivered.
I was confronted by a very long list of various types of Sikaflex and decided on 522 as it was listed as suitable for caravans..
What do others recommend as an adhesive/sealer?