"What also seems odd is that none of the solicitors we have contacted seem to know the process to deal with this, even those who draw up these Trusts themselves."
To be frank, it is likely only the very largest (and most expensive) of law firms that will have experience with a Deed of Surrender. It is likely that their charges may equal or be more than the amount that is in dispute here. And how likely would you be to recover that money from a person in prison?
If you're interested then there is a website called "The Legal 500" which lists the most respected solicitors, barristers, law firms, chambers etc in different fields.
Here is a list of solicitors in London that are recognised as being particularly experienced in this area of the law:
https://www.legal500.com/c/london/real-estate/residential-property/
There are also lists of those solicitors outside of London as well split up by different areas.
You may also wish to consult another website by the name of Chambers. They do a similar ranking of lawyers:
https://chambers.com/legal-rankings/real-estate-litigation-london-firms-1:663:11814:1
But seriously, these people generally charge really massive amounts of money.
"The terms of the Trust were that they both had to pay 50% each of the mortgage and cost of any home improvements."
Another approach might be instead of going for a Deed of Surrender to say that his partner has failed to pay his share of the mortgage as required to by the Declaration of Trust and so the missed payments will be deducted from whatever money he is due under the Declaration of Trust.
For example, you say that they bought the house in 2019. I don't know what mortgage he has been paying over that time but, for simplicity, let's assume that it is £1,000 per month.
In that case, his partner would be expected to pay £500 per month. Let's assume that the house is sold this year so that would mean that the mortgage has lasted five years.
This would mean that his partner had failed to pay £30,000 as his half share of the mortgage (£500 x 12 months x 5 years).
You might also want to add in any large expenditure on improvements (eg a new kitchen or bathroom).
Speak to a solicitor as to the best way of achieving this. It may be that your DS takes him to court to recover the £30,000 or however much it is (and costs as well).
His partner obviously does not have £30k so he can then get a restriction on the house title placed against the partner's share of the house.
"So the property cannot be sold unless my son agrees to give 40k ..."
This is true, however, if your DS gets a judgment against his partner for the outstanding mortgage payments over the last five years (£30k or whatever it is) then his partner will have to give your DS the amount that is owed to him.
So, it is likely that your DS will probably have to give this guy some money, but that will only be after deducting whatever the court awards him because the partner didn't pay his 50% of the mortgage.
Although, please, do speak to an experienced solicitor about all of this.