Capetown, I'm incredibly grateful for all your help - thank you so much for checking through the individual pages!
I notice FindMyPast has a burial record for Adam in 1827, stating his age as 73, which suggests a birth around 1754. If that's the case he would have been only 18 if he married Elizabeth Rachley/Rackley in 1772, not 21 as claimed in the Gloucestershire Marriage Allegations etc.
The burial record for Elizabeth listed on FindMyPast: 24th Nov 1837 (St Bartholomew, Aldsworth), states her age as 82 (born 1755), which would have made her just 17 in 1772! Either some miscalculaton going on, or they lied about their ages.
I've not yet found any other Jackson or Rackley with connections to Upton St.Leonards yet, but still looking....
Clearly however, it seems the Windrush Jacksons and those of my family were entirely separate, at least in the mid-1700's.