They're actually called "slip on spurs". They're a western thing and I think barouque/"Spanish" riders use something similar. I'm not sure how well they work on English boots, though, as the western ones I've seen sit lower down, not at the ankle like English ones.
I used to have dressage lessons from a Portugese classical rider (and one-time bull fighter!) and he always wore these. As I remember they sat almost on the join between the sole of the boot and the leather. Stayed very secure.
The Iberians use them a lot. I have a pair that I bought in Spain. As said above they sit on the join of the sole and the leather, so alot lower than the usual spurs but are very secure. I've used them with my jodphur boots and long boots with no problem. They are really handy as you can stick them in your pocket and pop them on and off as required. I've never seen any in the UK though sadly.
They slip between the boot and the heel and are very secure, so quick to get on and off. I think this must be more of a spanish/portugese way of riding at the moment. I was hoping that someone may have sold them in England but I have tried Inch's etc and all to no avail.
Thank you all for your help anyway.
x