If the solicitors have a copy it may be that it shows when the original was made and who witnessed it. A will is not invalidated merely because it cannot be located. Indeed, there's an extreme case where an eccentric old man made his daughter recite his will so often that she could remember what it said; he wanted her to be constantly aware of how generous he was being to her; and when he died no will could be found. The court ordered that the terms as she remembered them were binding and to be carried out.
What I'm thinking is that the witnesses, or one of them, or any person who saw the original when it was witnessed and signed, can give evidence of it. Being shown such a copy they can say that it is indeed a will which they witnessed etc. They may not remember the details of it, without seeing the copy now, but they'll be able to say that they witnessed it, as they can see, and therefore the copy would represent the original. In any case, the will's execution in due form would be presumed because the doctrine of ' omnia praesumuntur rite esse acta ' ought to apply ( that a document which is in regular form and of a type which the person would complete in the course of their duties, is correctly done) I'd have thought, even if such witnesses cannot be found or are dead.'.
It's worth exploring this possibility. The will, though not found, might then be admitted to probate.