Definitely "colt breaker" to my eyes. Interesting that he's listed his father as deceased (conflicts with earlier mentioned family stories about parents + sister dying in the Blitz)
freebmd.rootsweb.com shows:
WIlliam Oakley m. 1904 Aylesbury to Louisa Wilson
a possible older child for this couple, Gladys L(illian) Oakley b. 1906 and m. 1926 to a Herbert G Martin which would cover the witnesses.
Still cannot find anything concrete about Gordon or father George. It's looking quite likely that there was a George, colt/horse breaker/trader (or somehow otherwise engaged in that industry), possibly with some link to Newark, but almost certainly not with the surname of Mark(in/er)son.
The two documents to go after now, I would think:
Gordon's military records (I linked to the MOD a couple of posts up)
Any newspaper records for the 1949/50 case (copies may be held, possibly on microfilm, by either WYJS or in the British Library's newspaper collection - but I think the later is about to be moved so not a good time. Either way would probably require someone to visit in person).