Giles,
I'm sorry to hear that you've had so many issues! As you may or may not know, our company
acquired the rights to BV Commerce in December and have actively been developing a new version. Take a look at the
feature list and then take it for a test-drive with our
demo store.
Let me address each of your issues:
Originally Posted by: efficientlight 
Been with BV for many years but some of the limitations on export eg sales info / transaction lists etc are just too significant to continue. Can anyone help?
BV Commerce 2013 includes a feature that we call the "feed engine". The feed engine allows a novice developer to quickly and easily build text file exports from the system. There is an example order export for T-Hub that is included and could be adapted to suit your needs.
However, since you're using Sage, you should talk with Jenny Bailey at
Discover IT. I know that they have a BV Commerce integration with Sage. They are also a
BV Commerce partner, so you can be assured that they are competent with the platform.
Originally Posted by: efficientlight We have also never got a resolution to a Google product feed export since Google changed their google product algorithm ~2 years ago...! 2 years of no products listed on google shopping is an expensive problem to have with our website.
BV Commerce 2013 includes a new Google Shopping feed that meets the current feed standards.
Originally Posted by: efficientlight Some 5 years later we still struggle massively with the import tool on BV.
Yes, this tool definitely has issues. In BV Commerce 2013 it is replaced with a web-based version.
Originally Posted by: efficientlight We've been advised off upgrading our 5.4 version as the newer upgraded versions are "too buggy".
BV Commerce 5.7 and 5.8 are very stable, but if you're looking to upgrade I would wait for the 2013 version that will be released next month.