Final update on the helicomatic:
I actually did cause a little cosmetic damage undoing the lock ring with the pliers wrapped in a cloth, but to be honest thats ok with me. The hub came off easily then. At last I was able to replace the broken spoke that sparked this all off.
I also at this point gave the thing a good clean as it was filthy and cleaned the hub and spokes as well, being careful not to let any water (especially soapy water) go into any of the moving parts.
On closer inspection the free wheeling action was a bit gritty and uneven and even stopped a bit so:
I flushed it through with light oil (as per DaveM's idea - I used '3-in-1') which brought out a whole load of black stuff, but it also washed the grease from the bearings (which are not really separated from the ratchet mechanism) so that when I shook the thing it rattled.
So I opened the cone (the bit with the bearings in) over a tub a plastic tub. I did this by replacing the unit on the hub without the lockring and tapping (fairly hard) on the square protrusions on the cone lockring (visible in the third photo above) in a clock wise direction using a long brass screw which fit and a hammer (the nail I tried wasn't hard enough and the point broke). This was as per the instructions here:
helicomatic museum
Inside there were 2 sets of bearings and I kept these in separate trays and counted them to be sure not to loose any.
I cleaned the rest of the black crud from the ratchet mechanism and oiled it lightly with 3-in-1.
I wiped the places for the bearings off and applied a line of 'multipurpose No. 2 grease' and using a nail (for some reason this was slightly magnetic) I stuck the bearings back in place.
I greased (with the same grease) the threads for the cone lockring and carefully replaced the mechanism into the sprockets. This was a bit tricky because of the pawls and the bearings. I found the best way was putting two fingers into the center of the mechanism to grip it and then lowering it vertically into the 'back' of the sprockets and carefully wiggling it to get the pawls to fit between the contact points so that it dropped in. Just be careful of the exposed bearings on the other side.
I then screwed the cone lock ring back in and hammered it tight (there was a little pay between the mechanism and the sprockets until I did this).
I wiped as much grease/oil as possible from any exterior surfaces (the state of the bike as I got it demonstrated how greasy surfaces just get bunged up with abrasive crud so I wanted to avoid that problem as much as possible).
I greased the splines on the hub and slid the thing back in place, cleaned and greased the threads of the lock ring screwed it on and tightened it with the pliers wrapped in a cloth. Sorted!
It goes great now.
All I need to do now is overhaul the hubs (the front one is very clunky), clean and align the breaks, true the wheels, clean up the deralleur, rub down the rust spots and touch up the paint work and I'll have it tickerty-boo. £28 well spent...