Change outgoing repo on a hgsubversion checkout

Change outgoing repo on a hgsubversion checkout

For various convoluted reasons, I had to svnsync a subversion repo to be
local, before I could clone it with hgsubversion, rather than cloning it
straight from the 'source'.
Now that has worked successfully, (20secs vs 3hours then fail) I wish to
change the pull URI back to the original source, since it works fine under
normal circumstances. However, if i change the .hg/hgrc URI back to the
original source and do a hg pull to check it's worked, I get a abort:
unable to operate on unrelated repository message. Obviously, the 2 svn
repos are identical, but even the --force flag doesn't seem to convince
hg(subversion) that they are identical.
I also tried running a hg convert on the local svn repo, however when i
went to pull from the original source, it seems that it hadn't retained
the history (despite it being there already) and decided to pull
everything again.
Anyone got any ideas?