I will try to assist you with this as no one else has yet picked it up. I have only just come online.
Your stepfather cannot avoid this forever. Once the will has been admitted to probate (a court process) which is essential to transfer the house into his name, you can obtain a copy of the will from the probate registry and the fee for getting that is £10.
Here is the link. https://www.gov.uk/search-will-probate
You will then be able to see the provisions were in the will.
Until then, even beneficiaries have no absolute right to see a will. If you think that your stepfather is up to something untoward, which he should not be, then you can always make an application to court for what is known as “pre-action disclosure” of the will and you can ask the court to award costs against your stepfather. You will have to write to him to tell him what you propose otherwise if you just go to court, although you will probably get the order, you may not recover the costs of doing so. A solicitors letter threatening that application may make him see sense.
In cases like this, I always tell people like your stepfather to simply hand over a copy of the will because all being secretive about it does is raise suspicions that something is untoward and as it’s going to become a public document in the end anyway, there is nothing achieved by being secretive.
Can I clarify anything for you?
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