Thanks for the reply maths...
I have no clue about the legal aspects of this really but I had to complete Land Registry documents when we took out the joint mortgage on the remaining balance of debt owed on the house i.e. he retained his equity already gained in the property by adding me as a 33% owner of the house once the mortgage was paid off. This was stated on a form we signed in person at the Solicitors. As I stated I did get a bill for the 3% SDLT from the Solicitor at the time, but as I had sold my main residence in 2017 I fell within the 3 year transitional period of not having to pay the new higher rate. The Solicitor didn't actually know this and then wrote back to retract the bill and sent the HMRC guidance note on it saying that she agreed with me and no SDLT was payable.
Then when we ended the joint mortgage I had to complete an LR1 form to, I think "dispose" of my interest in the house with Land Registry and be taken back off the deeds to the property. My ex-partner paid me a mutually agreed sum of money for what I had put into the house - this was not organised through a Solicitor, just an arrangement between ourselves.
I have contacted a Solicitor today who is a SDLT expert, especially around the higher rate rules, and he agreed to take me on as a client for an initial fee of £840 to tell me which points in the guidance note to refer to when making a case for not paying it; he then wanted £275 an hour to compile a written report to give to the conveyancing Solicitor who would complete the SDLT1 form on a new purchase.
I do not really want to pay such fees as I think the information is available within the guidance note for me to compile my own case. I am a qualified chartered accountant, but obviously this is not something I am familiar with. My friend who advises people on tax efficiency on pensions and investments etc. says he can charge £6k+ to someone who could actually find the information themselves on the internet if they had the confidence to do it - which many don't apparently.
I just wanted to get some other opinions before I start to invest lots of time into researching the very specific rules surrounding this