Advice Needed

I am not sure if I have posted in the best section,

In 2006 I purchased a mortgaged property with my now Ex-wife and her Father acting as guarantor.
In late 2009, we parted ways, and it was agreed through Solicitor’s for me to vacate the property and be removed from the mortgage. No monies would be paid to me, and this was agreed.
The Day I moved out, December 26th 2009, I contacted NatWest Mortgages, and gave them my new living address (My Parents), informed them of the situation, and told them my mobile number was still the same. They confirmed they had “Updated there records”
In 2014, I get a letter through the post from a Shoosmith’s solicitors, saying that I owe NatWest Mortgages a sum of £30,000 for the shortfall of the mortgage. As you can imagine, this came as quite a shock.
I contact Shoosmith’s, and asked them for more information. They declined to send any.
I then did some research via the internet, and found out that the house had been actually been repossessed.
I was again sent a letter from Shoosmith’s, the same default / Template letter, asking to get in contact to arrange payment or a suitable monthly contribution to the debt. I again asked for more information. This was declined.
This has been repeated and repeated to this day, the same default / Template letter, attached with an expenditure form for me to complete. Everytime I do make contact, I do not acknowledge the debt, and to this day I am still waiting for information. Date of Repossession, Auctioned price, was there a Judgement of money etc.
I have also lodged a complaint two years ago with Shoosmith’s. My complaint was based on the bombardment of the same repeated letter, never taking into account my communicated response.
This obviously appeared on my credit file, but since November 2017, has now been removed.
I have lived everyday with this hanging over me, as you can imagine this has brought great strain and health issues to me and partner.
We recently spoke with a Mortgage Broker as we would like to obtain a joint mortgage with my new partner. This has been approved in principle.
I privately rent and have no vehicle so new valued assets. He is concerned if I now appear on the mortgage database, they could then take our new joint property from us.
My Mortgage Broker did speak to his legal team, and they said they were surprised that due to it being negative equity, that they were still chasing after all this time.
I am after some advice on the situation, I should have maybe asked this a long time ago rather than just trying to find a situation on this forum similar to mine.
Should I go ahead with the mortgage, and just continue to exchange letter’s? Shall I wait until after 12 years? Any help or reassurance would be greatly wonderful, as if thinking about buying a house is stressful enough, I feel so very guilty that I am almost punishing my partner due to this.

Thank you for reading. Charlie.