I'm pleased to report that everything went perfectly well and there are no more leaks. Hoorah!
Unfortunately, the low-rev problems persist and I'm buggered if I can resolve the issue. I just don't know what else I can try - I feel like I've replaced everything that matters on the bike and all to no avail.
That doesn't mean I'm gonna give in though! Oh no. Winter's approaching and that's when I tend to spend a lot of my leisure time in the garage, so maybe I'll solve the mystery then.