Jenny, I am so very sorry to hear this - My sincere condolences xx
When my OH's father passed away a few months ago, his mum contacted her solicitor for initial advice - The best advice she received was that there is no need to rush.
The solicitors advised that she contact the local tax office and start from there - this particular tax office connected with the rest of the council and informed every department of his passing and were extremely helpful.
Other general things to consider apart from pensions and savings accounts are things such as TV licence, direct debits, email accounts, club subscriptions that he may have belonged to.
Take time to grieve first Jenny, and take up any offers of help from your friends and family right now - I know my MIL thought she could cope and sort it all out on her own but it does help to have people around to help you if you can bear it. Such sad news, sorry Jenny xx