Thanks for the help, folks, very much appreciated.
I have seen many other trees on Ancestry that make these connections, and I'm in touch with some of these folks. However, the documentary evidence they are using is the same as I have, and they don't have any family papers or anecdotes to move the case from strongly circumstantial to proven.
I have made really good progress on Ralph's history, I have his discharge papers and a lot of family stories to begin with, got his pension records, and more importantly i have the published history of the 8th Btn. Royal West Kent's so I can trace his movements almost daily across France and Flanders. I hope to get to Kew this year to look at the unit war diaries to see if there's more there.
I looked for Thomas' service records and they are gone, bar his medal index card, which adds nothing really new. It's a shame, as a list of next of kin on the enlistment papers would have clinched it for me. I have ordered his Birth certificate, but I'm sure it will say Wadhurst.
The 1911 census return would have been completed by Belhaven, so perhaps Crawley is an error on his part. We'll never know.
I looked for a death certificate for Thomas, as a date of birth or next of kin on that might have helped, but I cannot find one. I understood that the army completed these for soldiers killed in the western theatre, and I have looked from 1915 up to 1920 without success in the BMD indexes. I wonder if the fact that his body was never found is a factor here - any advice on this would be appreciated.
I have got hold of the digitised war diaries for the 13th Btn, the Kensingtons from the PRO, and Thomas is mentioned several times in it in lists of men participating in various actions and as KIA on a casualty liust for Aubers ridge. This adds a lot of context and detail if this is my relative.
The local paper angle is the one remaining lead I can think of, I think I'll look into that next.
Thanks again for all your help
John