Thank you for all the comments here.
We lived in a family house for 23 years, then moved here and tenanted the family home but have no plans to sell that within the three year window that we could use it for SDLT purposes as we have downsized and that house now provides a good pension.
We originally bought the current home as a BTL in 2016 with a view to downsizing and retiring here, which have now done.
However, having lived here for some time have now decided we want something different and are selling to free up funds to buy a new home without being part of a property chain.
We moved everything to this address when we moved in, including doctors, cars, etc per the book.
As we have another property locally to us here we can use (another BTL we bought in 2000), then it seemed sensible to use that rather than pay rent elsewhere.
However, as I started to consider the tax implications I thought it wise to seek clarification. It all used to be so simple (by contrast) until all the CGT and SDLT changes came in.
We may we rent something instead which seems to make the case simpler as the tax manual specifically states that an AST is disregarded, but thanks to the participants here I can make that decision with greater awareness.
It sounds like you just have to hope you don't get investigated and need defend your actions, but I guess that's true about many aspects of life, and being prepared with good documentation etc., is key.