Andrea/Jan
Thank you both very much for your "rapid response" to my query, it is much appreciated.
If I may, I'll answer all your points basically in the order you responded to:
1. Ages on marriage cert GTB and MAS was "FULL" - not a lot of help was the rector of St Marks that day

2. Parents were David BRAZIER Publican and Caleb SMITH Butcher.
3. Witnesses were Caleb SMITH, James ? SMITH and Alice Wyborn.
4. Most likely birth that I found for MAS was GRO: Sep 1865 Battle 2b/40

5. Have found no references to the death of GTB anywhere

Sadly grandparents are not around to ask, but they did live in GTB's house in Tower Road, Worthing for a short time when I was a kid!
Many thanks for the 1901 Census - it's spot on. Kate Mills BRAZIER, by the way, got the Mills bit from her mother Kate MILLS (1844-1913) who married David BRAZIER (1840-1915) in Southwick
The 1871 Census for the SMITH family looks good too as it includes at least one of the witnesses at the GTB/MAS marriage

I agree, I'd rather cycle from Battle to Bexhill rather than to Brighton, so I agree your conclusion that the Caleb/Eliza MITTEN union the most likely
GTB's death is still a mystery but I'll keep beavering away at it. Once again my indebted thanks to you both

Regards
Ken