Can anyone point me in the direction of any legislation that imposes a speed limit, by class of vehicle at roundabouts ?
To make this clear and we understand are you asking "Is there an actual speed limit imposed on a roundabout? "
I would say that if the road has a speed limit of say 50 and there is no reduced limit imposed before the roundabout then I would have to say NO .I have never heard of a statutary speed limit on any roundabout IE your in a 50 limt and all roundabouts are 30…
Same as the approach road limit unless signs say other
Nope,
Its as fast as you can go within the speed limit of the road.
Or as plod would put it, a sensible speed for the type / tightness of roundabout and direction of travel.
That reminds me of a copper who pulled me once, quite a few years ago now.
“Do you know why I stopped you?”
No, I aint got a clue, but you are probably going to tell me 
“You were going a bit fast around Howden roundabout”
Was I? Then how come my load of sheeted baled paper is still on the truck? Have you been driving trucks long officer?.
“I can’t drive a truck, I don’t have an HGV licence”
Obviously. Have a nice day eh! 