In case anyone is interested, or it is useful to others in the future, I actually managed to track down an XL 650b Stumpy Evo to take a spin round the local streets today. The geometry is pretty much the same as the 29er but with a lower stack height on account of the smaller wheels.
The XL wasn’t bad, and in terms of actual seating and standing position it was the more comfortable bike of the two, with a shorter stem even more so. With the frame being so low slung there was also acres of standover on the bike, it felt really nice and low. However what did put me off was the wheelbase on the XL bike, it felt really cumbersome to throw about and turn tight, ok it was only on the street and at best chucking it around on some lumps and bumps in the local park, but it was enough that I feel it would be a handful in the tight stuff. Overall I think I’m perhaps decided I just can’t get the Stumpy to work for me size wise, what I really need is the XL seating position with the L wheelbase. So on to plan B & C, which would either be an Enduro or the new Transition Smuggler.