It's a pity that Barmaid hasn't come back to this, as i for one would like to have known more about this.
The principle that she talks about - dependent relative revocation - means that if you purport to revoke a will on the basis of a new will which you make is valid, but this new will is not valid, then the old will stands and is not revoked.
There would seem to be some evidence of this in your aunt's case - she revoked her existing will by destruction on the basis of an intention to make another will.
You are definitely going to need legal advice for this one - oh, and I trust that someone has a copy of the destroyed will?