If I can just add, a will is revoked by writing a new one which specifically refers to revoking "all previous testamentary documents". Just writing a new one will not revoke the old one unless it specifically says so (generally speaking). However a later will will revoke an earlier will to the extent that a later will is inconsistent with the earlier will but that can be quite complicated construing two documents together.
A will can also be revoked by destruction, either by the testator or by someone else in his presence and at his direction, with the intention to revoke.
Only problem is, if you revoke the will (by phoning the solicitor and asking the solicitor to draw up a document revoking the will) you will then die intestate. (EG an earlier will would not be revived by the revocation of the later will). Best to revoke a will by executing a new one including a revocation clause.