When I use the .exe to open the game and get the splash menu with the skull in the centre and the six options around the side, the top left selection now says 'install' rather than allowing me to start the game.

I've had the game installed for 6 months or so already and its not been a problem before. I did recently upgrade to Windows 10, but not sure if this is connected or if I've somehow managed to delete some registry info separately or something.

Is there anyway I can get around this splash menu and go straight to the game? Or something I can do to reinstate the 'start game' option in this menu without doing a full reinstall? I don't want to lose my save games!

Go to your game folder and make a backup copy -- move it somewhere else -- of your 'save' folder. Now uninstall and reinstall the game, then move the backed up save folder to the new installation.

That could be the culprit -- upgrades always carry the risk of breaking something. That's why installing from scratch is generally the safer approach even though it's more annoying and time consuming.

Anyway, go to your BG2 installation folder and copy your saved game folder somewhere else; if you're using custom portraits and/or soundsets, copy the respective folders as well. Next, check if Windows still views the game as installed by going to Programs & Features (or whatever it's called in Win10); if BG2 still shows as installed, uninstall it.

Regardless of whether or not the game was shown as installed, delete the BG2 installation folder and reinstall the game. Finally, copy your saved game folder and - if applicable - your portraits and/or voice set folder(s) back into BG2's installation folder; if prompted to overwrite, confirm and overwrite. Just read that after I posted, so...

Make a backup of your complete BG2 installation folder; check if Windows still thinks it's installed (see above) and uninstall it in case you can. Reinstall BG2, then delete the content of the installation folder (not the folder itself), and copy the content of the backup folder into the installation folder. This should restore the BGT installation.

I really don't know what Windows 10 upgrade does, so I can't say for sure. Try the second method given by notsofastmyboy, it may work.