Resurrecting an old thread again, but thanks for this.
Both sides had started to leak water on my 200, which in turn managed to run down into the boot (trunk) and was dripping inside off of the boot (trunk) hinges.
Boot seal, rear quarter window seals, rear roof seal, all pulled off and inspected.
All put back on again and sealed with a thin layer of windscreen sealant.
Turned out to be these things, upon inspection the internal rubbers go flat with age and allow seepage.
Thin layer windscreen sealant, reattach to car ...
... Mother Nature performed her rain test a couple of times over the weekend and I still have a dry boot (trunk)
Seems that I took the long route but got there eventually.
Feeling confident enough now to put the boot (trunk) carpet back in.
BTW. If anyone else feels the need to pull their rubber trims off, save yourselves some hassle and buy a set of trim removal tools, much better than a spoon and only about Ł7 on eBay