Thanks Arthurk and Davedrave.
I had a mooch on that eBay link you sent and the description on the images of other Sibford cards have the same flourish on the S just like mine does and this is attributed to Percy Simms.
Googling further I found that Simms was a Chipping Norton photographer and a rival of his, Frank Packer and latterly his son Basil, took over Simms' studio and all of the Packers work is now digitised
https://heritagesearch.oxfordshire.gov.uk/search/all:images/0_50/all/score_desc/frank%20packerBoth Simms and Packer worked from 1910 onwards, so the suggestion that the card is not pre-1903 seems well founded
Many thanks for all the research carried out!