"...as there are no living relatives or descendants left in Colin’s family."
This is the important point. You mention that there are no living aunts/uncles.
Did those aunts/uncles have children themselves? Are any of those children (or grandchildren) still alive?
If so then they inherit, otherwise, as you say, it goes to the Crown.
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Secondly, did Colin ever formally adopt Laura under the Adoption Act 1976 (rather unlikely, but it's a possibility)?
If so then Laura is Colin's daughter for the purposes of the intestacy rules and can inherit.
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Otherwise it will go to the Crown.
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If Colin doesn't write a will then Laura still has two possible ways of getting the money.
The first is by making a claim under the Inheritance (Provision for Family and Dependants) Act 1975 as she is somebody who "was treated by the deceased as a child of the family"
If Laura is still a child when Colin dies then she will have a stronger case but, even as an adult child, it is still possible to make a claim.
There was a case in the High Court a few years ago on just this issue.
Higgins v Morgan & Ors [2021] EWHC 2846 (Ch)
Barrie Higgins was the stepson of Stewart Higgins.
Stewart died intestate in 2017. This meant that everything went to Stewart's nephews and nieces rather than his stepson.
However, prior to his death Stewart had bought a house for his stepdaughter (Barrie's full sister) and Barrie stated that Stewart had promised to leave an equal value in his will to him. He also said that Stewart helped him out over the years when he was short of money.
The court ruled that there are four stages to determining if a stepchild can make a claim:
1 Can they demonstrate a need for maintenance and, if so, to what extent.
2 Can they demonstrate that there is "something more" than just the fact of there being the stepfather / stepson relationship.
3 If the above are satisfied then was a reasonable financial provision made
4 If there wasn't a reasonable financial provision then what should it be
The court ended up awarding Barrie £41k from an estate worth £260k.
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The other way would be by something called proprietary estoppel. This is where a person has promised something and the other person has relied on that promise but then the original person has gone back on their promise.
This can be seen in family farms or family businesses. For example, if a parent says to a child "Do all the work on this farm and then, when I die, everything will be yours" but turns out to leave the farm to somebody else. In this situation, the child can bring a claim against the estate.
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But, given the huge hassle of doing all of that, it is much better if Colin just writes a will.
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"Colin doesn’t want to spend the money re-writing his will."
If the will is as simple as you say then he doesn't need to spend money re-writing it.
Just type up a new will copying everything word for word with the exception that you replace any reference to "my son John" with "my step daughter Laura".
Then get two people to witness his signature and he has a new will for the cost of printing out a couple of pages on his printer.