A few thoughts:
1. Is father still alive? If yes, he should make a new Will. If not.....
2. Do a thorough search of wherever he kept all his important papers. If that yields nothing then
3. Contact the last solicitor he had dealings with, that you know of, and enquire if they have his will. If that yields nothing then
4. Most people don't use a lot of solicitors over the course of their lifetime so contact those that you know of and make the same enquiry. If that yields nothing then
5. Advertise (newspapers/ Law Society Gazette) for any information. If that yields nothing then
Perhaps your father destroyed his Will?
If you are aware of what was in the last Will you are aware of and if it was an even split between usual suspects/ follows general intestacy situation, consider an application for a Grant of Administration Intestate.
mf