hangarshuffle, you are quite correct about the old Chard canal at Merryfield. I used to explore the old aerodrome in the '50s-'70s and well remember the site!
Here is the canal on an old OS map, with Merryfield's runways superimposed:
The canal used to run between the 2 yellow markers on this photo:
The other line feature, to the west of the aerodrome, is the old line along which the 'Chard Snail' would chuff its way from Taunton.