If there is a will, the executors generally have to apply for probate (unless there is less money than probate limit, which is quite low).
You can check the probate register, and when estate is registered, the will is public property and you can access it for a fee (£10 I think).
Until then however, you have no way of knowing if there is a will, apart from asking the son, or ringing around local solicitors, but as you are not next of kin, that may look odd.
When did GU die? It can take time for probate and until then the executors cannot give any money out. It is possible you are in the will and just haven't had a payout. Equally, your GU may have been great for saying things to people, but not actioning them. Maybe he never updated an old will, or even had a will?
A cousin of mine died a couple of years ago. Everytime he visited his solicitors (almost annually it seems) he would visit another cousin locally and she heard many versions of where his money was going, including all to friends, and all to cousins' DC. The latter was what was in the actual will, written about 4yrs before he died!