Thanks for all the comments.
.I've always had it drilled into me that if you want to improve topline/encourage correct muscle development you needed to use some sort of gadget to do this be it a pessoa, sidereins or similar - in fact I don't recall seeing anyone at my yard simply lunging their horse in a bridle or cavesson and nothing else.

to those people!
I'm working him on the lowest setting to encourage him to stretch down but he's not really stretching - he's not fighting it but just going round with his head up slightly (he has a short neck and therefore a pretty low head carriage anyway) and his nose poking out.
I'm not sure if he just needs to get used to the aid or if I need to adjust the settings to asking him to come down more or what?
What I do is use body language as much as poss, to get the stretch I stick my own head out ( yes I look like a werido when I lunge

) , If they don't respond I encourage with the lunge line a little.

Sounds like a great place to ride! Poles will also help for sure
