I have a client who has to take one leg off the floor after a standing for about 20 minutes, to get his circulation going again.
I vaguely remember that I had a tribunal a long time ago where the members thought that standing on one leg only had to be discounted. But is there actually any caselaw on this?

Can't see any. The only case law I can find on standing is R(IB) 6/04 which makes the point that it does not mean standing stock-still but that the small movements (shifting weight, shuffling feet) everyone except a guardsman makes are to be disregarded, and implies that "moving around" is more than this.