Sam - sorry but it seems I've slightly misled you. I looked up that ULSA reference this afternoon. It's an article called "Free Diving" by Bob Greenwood in ULSA Review 13 (November 1974) pages 18-20. I's only a general article about free diving - not specifically about the Rowten sumps. The front cover of the journal has three monochrome images of a free diver at Rowten sumps - but these photographs say more about 70s caving attire than about the sumps themselves. It's an interesting article but I wouldn't go out of your way to root it out as it wouldn't tell you a great deal about Rowten.
I've not been there for a while but the following may help. All the sumps are bedding sumps, so nice and wide - but this means you can get lost if you somehow lose the line. As I remember the first sump going upstream may only be a metre or so long, with a thin airspace so you can talk to people at base. It surfaces in a nice airbell with thigh deep water covering all the floor. The second sump is only just sumped, is 3 or 4 metres long and has a nice flat roof; as free dives go it's one of the easiest, surfacing suddenly in another nice large airbell with deep water (where you can stand up half submerged comfortably). The 3rd sump is much longer (at least 8 metres). It starts off shallow and deepens almost imperceptibly so that there is a steep ascent of a metre or so at the upstream end. The roof is flat but just before the ascent you'll encounter a lower area - this is normal (it doesn't mean its shingled up). It is here where you might find a bit of old fencing or suchlike lurking - so take care!
Bon plongee, as our French colleagues say.