Mark,
It's amazing how very small our cars are inside, compared to modern ones.
I managed to squeeze two OMP 'Eco's into the Sb - they do an even cheaper one, the TR-S (!!!) that is side mounted.
An alternative would be a Kirkey seat, as in Binman's SuperSix. They look so cool, I'm amazed that Vinnie doesn't have four.
They also look uncomfortable, but 3000miles in the 10CR says they are comfortable.
You need a classic style, narrow backed seat.
Acording to a rather old Europa Specialists Spares catalogue:
Cobra sell a 'Classic' (490mms wide), and a 'Cub' (445 at the seat squab, 340mm at the top of the back).
Corbeau sell another 'Classic', an 'Alpine' and a 'Clubman' that all lack the shoulder extensions that make it difficult to fit.
Intatrim sell six different narrow back models
Richfield sell five that are less than 500mm wide at the base and less than 400 at the back top.
My 2002 edition includes a sizes chart for each manufacturer, but see:
http://www.europaspares.com/
John