I think I have found what I need.
The Caterham supplied Billys are not adjustable but you can get a kit to convert them for ride height adjustment. No damping adjustment though. That just makes them much more expensive than comparable items from Gaz, Protech etc that are adjustable.
I am leaning towards Gaz. I have had two sets before and are very good value, last well and look good. Protechs look great and are all alloy although do have a much larger eye surround for the bolt holes than steel items so may need some 'adjustment' (grinding off!)
The consensus on lengths between centres of eyes/ top of rear (to suit my LA car) are:
Front: 315mm
Rear: 440mm
Just as an FYI and get all my options on the table, I will also go with these springs rates:
Front: 275lb
Rear: 140lb
.. and if I dont like them, springs are only about a tenner each.