You have done the right thing. See what his response is.
In the meantime, a good tip will be to look for father's will (if there is one - I know SM said it would be divided three ways but sometimes people misunderstand how the law works - she may have thought that it goes 3 ways and not actually made a Will). Very often, married couples make Wills together. Even if they do not make Wills together, they very often use the same solicitors. Often a Will will give clues as to who drew it up. If dad did not leave a Will, try and recall the solicitors they may or may not have used. Once you have identified the firm, it is worth a call to the Probate Dept to ascertain whether they have a Will (they may not tell you though). It is also worth doing a search with certainty.co.uk. If you cannot identify the solicitor, it is worth giving all the local ones a call to see if they have a WIll.
If he ignores the letter, there are various legal options you have which I will deal with at a later date.
My view is to attempt to ascertain whether there is a Will first and who the executors are. If he wont respond you probably need to escalate this to a letter before action. Incidentally, I would also copy the letter to the aussie dweller.
Keep me updated. (And if you need to discuss directly, there are enough peeps on here who know how to contact me).