Well, Leigh Baxters were friendly but no help... It wasn't a size in one of the boxes so that's that..
Luckily another customer was listening and pointed me to P & A Hydraulics in Rochford.
Initial check was whether it was a size in one of the kit boxes.. again, no joy, as expected
But I was dealing with somebody who knows his trade... his next comment was, that's a standard id but non-standard cross-section. standard section would be a couple of thou larger.. PERFECT
I had read on forums how the original one from RockShox was if anything a little small in it's outer diameter and so, maintaining the id but increasing the section slightly to 'standard' size should help matters.
So, he's ordering me in 10 for tomorrow at about 10p each
Just gotta order the oils online now as these places don't do the grade I need in the low quantities.