Wow! Thanks everybody - you were busy whilst I was out last night!
There's a baptism in FamilySearch for William Henry John Gilbert 29 Jun 1882 at Sheerness
https://www.familysearch.org/ark:/61903/1:1:NTPK-17PHis father is indeed Henry Twisden Gilbert, mother is Emma Gilbert. Unfortunately this doesn't fit with 'our' William stating that his father's name was William on his marriage certificate.
Regarding Colin... The great aunt of the lady I am working with has told her family stories about 'Colin Gilbert'. Apparently the brother of William, he served on the Cutty Sark at one time and was the last person to be flogged with the cat-o-nine-tails... There is no evidence of this and someone has even checked the crew lists and not found a Colin Gilbert who served on that ship. I doubt the aunt would have fabricated Colin - but it seems like she may have got her wires crossed somewhere along the line! It was also said that William and Violet's son, Colin, was named after his mysterious uncle.
The Colin that was born in 1893 to Alfred GILBERT and Emma JOLLIFFE is 'my' Colin (although I wasn't really interested in him until now - he was the husband of my grandfather's sister - Hilda Maud Grant). He was difficult to trace because he is sometimes called COLIN GILBERT, or COLIN EDMUND GILBERT, or EDMUND COLIN GILBERT, or COLIN E GILBERT. He was born in 1893 - Penkridge Staffs and lived in Leicester for the latter part of his life. Fortunately Colin and Hilda's first child, Phylis, was born in Boston, Lincs. This was where Colin's parents were living and was the proof that connected my Colin with Alfred and Emma.
The tree on Ancestry has got them muddled up and needs changing. I think there are other similar messes on FamilySearch!
Re the 1891 census - there's nothing obvious at the moment.
That 1911 Census looked tasty but alas it seems it isn't 'our' William.
I'd heard about the demise of William - I cycle occasionally and it's always a worry...
Well - onwards and upwards. Thanks again to you all for looking at this!