It's easy to access and a pretty small area, but I only found one individual today (though it was windy). If the other site you were thinking of is near Frampton Cotterell, then I believe they are in better numbers there, though I've not been there to look.
If you want to try the Stoke Park site PM me, and I'll help you look for one of them if I'm free. The site in general is great for variety of odonata, though it's a small site so numbers of individuals isn't that high. I've had 24 species within about 100m of each other in the last few years. 12 spp noted on the wing so far on patch this year, but Scarce-blue-tail the only notable one so far.