What do you do about gutter joints when a wall isn't straight? In
other words it consists of 2 sections joined at an angle of 170 degs
instead of 180. Unless there's some kind of adjustable straight
joint, one would appear to be buggered....

I really, really wonder if the wotsits rotting my wotsit as sometimes I
can barely remember yesterday but I recalled your question was asked
before. Google then did the rest:
https://groups.google.com/forum/?hl=en&fromgroups#!topic/uk.d-i-y/pHx2L97sH7s

Subject of course to Andrew Mawson or others providing an update.
--
Robin
reply to address is (meant to be) valid