I've now got William's death certificate: the informant was C J Pinching, who appears on the 1861 census as Charles John Pinching, a surgeon - he was present at the death.
The cause of death is:
"Disease of the brain of long standing. Bronchitis 5 days."
Poor bloke!
Obviously this is one of those fabulously flabby Victorian causes of death (at least it wasn't phthysis...) - I suppose it could be brain cancer? I'm also wondering if it euphemistic for a mental illness, as in all those people in Victorian novels suffering from "brain fever".
The thing is, if this disease was of long standing, why hadn't he done his will? Or bearing in mind the above court summons, do we suppose there was a will but his sons felt that their stepmother had influenced it and been prejudicial to them?
The certificate is a typed one rather than a copy of the actual handwritten one - not really sure why, other than that there's an error because he was initially registered as William Clements aged 52, which has been corrected to 51. So there's addenda down the side.