That is promising news Chris. A quick check shows that Lechlade Marina is 1500m upstream of the Trout Inn slip - almost no problem rowing/sailing downhill apart from another lock. 
However, I don't see a large area for caching cars and trailers prior to the Beale car+trailer shuffle. The New Inn pub has a large grass garden by the river and it 
might be possible to store trailers there for a few hours. I stress might, because G-Maps shows rocks near the pub car park to keep cars off the grass. 
Nothing a 4x4 and rope cannot move 

 Bring your tractor John? 

cheers
Paul