In the end I measured carefully and took the gamble and everything fitted perfectly. Thanks for the advice on the tar, that happens when it rains heavily so it may simply be old build up in the chimney. But you are right that we don't know so we will get that looked into before next winter. All of the wood we've been using is fully dry so hopefully if it is an issue, it's an old one that can be fixed. As a stove we really love it, on milder nights with less draw it can take a while to really get going but it's incredibly efficient in winter.