@tommmmmmm @Howard @aggi
Thanks, just the sort of thing I needed to get me started on thinking and researching.
We live in Scotland so things are slightly different (no stamp duty, it's LBTT here). What I'm considering appears to be a transfer of mortgage debt and a gift of equity. It would appear that the transferred share of mortgage is below the LBTT threshold so doesn't incur a charge but still must be reported.
Fiancée doesn't own property so no issues there.
I'm considering remortgaging because there are much better deals available than we currently have (the property having gained value thus my equity having increased).