Ok. First off does the flat complex have a warden, that's a good place to start. It might be as well to try to pull together some kind of informal proof of who you are and what your relationship is (have your aunt and uncle got pictures of you? can he confirm to the warden/social services who you are. Its not an uncommon situation and can be dealt with. The GP is also a good one to try to talk to but may not be able to do much under the current circs.
Yes talk to social services about your uncle. The hospital social services department and maybe the occupational therapy department should also be making enquiries regarding your uncle's wellbeing.
Social services and the hospital may not be allowed to tell you much but you can tell them stuff an express your concerns. The hospital will be likely to be able to tell you more, especially of they have met your uncle and can see how vague he is.Yes do try to find out where the wills are. There must be more than one, you can't do joint wills.
Practically, what we did for my late Mum was to put a sign up by the phone in her flat saying "in case of emergency contact" and the names and contact detail of us, her daughters. When she went out, before she became housebound, she carried the same info in her bag (actually I do this as well and I am only middle aged and fit but I am widowed and live alone) The same information was also at her GP and social services but she had to give permission for this to happen.
Once you have phoned around and gathered and given the info, could you do one flying visit to your uncle to tie up the loose ends?
If you want anymore info, do shout.