Thanks chaps !
All opinions have genuinely helped ( and thanks to Ratrod for that very interesting link ) . :D
In the end I have gone for the 10" Greenback.
The reasoning ( based on your opinions and conclusions / data that those opinions lead to ) was that :
1) I have tried ( and have in reserve ) Jensens for Fender(y) stuff - and for that purpose, the bright clear slightly 'stiff' sound seemed to bring out the characteristic 'twang' and bite that seemed appropriate for my purposes at the time ( 50s & 60s Soul / Rock'n'Roll ). To have a smoother alternative to my existing 'wall of glass' - Ratrod's excellent Jensen link seemed to suggest the need for a Jensen P10R - and as Watford do not have a 16 ohm version - the P10Q. It seemed like a lot of cash for more of the same.
2) Similarly Fender seem to deploy the Classic 30 in things like the 'Tone Master' cab - and Jazz Guitarist Adrian Ingram has one in his Matamp Custon. This might have at first seemed like a qualifier, but upon reflection seemed to indicate that whilst being perhaps more flexible - it was still being chosen to best present that cleaner, tighter sound.
3) As my Classic 20 is the only Valve amp I possess that does not have 6L6s - and is voiced uber clean, perhaps ( after some reflection ) a marked contrast seemed in order. Also if this worked out, if I ever got rich my 4 x 12 Matamp cab might one day be a candidate for at least two of these.
Ultimately I trust your ears chaps, and the more I read your posts - the more I saw common ground between your opinions.
So I ended up second in line behind gwEm at the Watford market stall this afternoon.
I am certainly looking forward to testing out this 'new' type of speaker, being as I am already surrounded by Jensens, Eminence Fender specials and the ubiquitous GT12-75s.
Thanks again ! :D