Notch, hope you dont mind me asking, but do you know of a supplier in the UK for S grade ply? I know its used for say, premium toy manufacturing and so on, but its like hens teeth to get ahold of so I've had to make do with B/BB most of the time. I even had one timber merchant chuckle when I asked for it..!
I havent used it for donkeys!
I did do a quote a year ago for a job and from memory Lathams stocked some in 18mm
I tried Meyers, which I seem to remember didnt stock it.
There is a major independent importer in Essex that might be worth a try:
http://www.dhhtimber.co.uk