I then tried Antonio's method of placing a 1.5 ohm resistor across the gap
and connecting the signal genny in parallel to it running at 1Khz square
wave output. The secondary and top load were connected and back in place,
whilst the NST and Terry were disconnected. The scope was placed across the
primary coil.
The resultant trace can be seen here:
http://homepage.ntlworld.com/follies/temp/scope.jpg
It looks like it may be correct as I can see what I presume to be the
ringdown, but I remember seeing a scope trace somewhere else on the web
where the abrupt sharp Y axis rises that I have, were replaced with more
gradual 'ringup' shapes, so I wondered what anyone else on here thought.