There are two possible sizes of Sprat and Winkle couplings to choose from to use with 00 gauge stock.
On the left is the 4mm scale designated one which is for remote uncoupling and shunting. So, after looking at the original type seems somewhat oversize.
The option is to use the 3/4mm scale ones which are a lot smaller and neater.
The 4mm size is shown attached to a Bachmann Toad Brake Van which to my mind looks far too big.
So, another order to Wizard Models resulted in an etch of 3/4mm couplings which are a lot neater and enable closer coupling. As St Mewan is a small layout ,tight curves are not an issue so reduced distance between vehicles is possible.
This is my attempt at fitting the couplings. instead of assembling on a mounting plate some .4mm holes to thread the wire through were drilled in the wagon base.