Further to my above post – if your late father used a solicitor to draw up his Will, then it is highly likely that any subsequent Will was made with the aid of professional help. Most (if not all) solicitors will want to keep a copy of any Will made with their assistance, it could be that he used one of those Will writing services who advertise in many local papers.
If his Will was self-made, then I suggest you talk to those who might have been witnesses to the Will, who may remember witnessing the Will.
It is possible that he deposited any later Will with his bank.
Other than to keep searching for the later Will – I would suggest you seek advice as to your legal responsibilities (being a joint executor) - how long you can reasonably delay in distributing your late father’s assets. And what would happen should a later Will be discovered, once those assets have been distributed.