Any question answered...

If it's really 'a few mm' you can stick a BB spacer or singlespeed conversion kit spacer behind the sprocket. They are available in 1mm increments from 1mm upwards. If you want something with a wide foot, try Trialtech, nicely made sprockets with a 10mm base
http://www.tartybikes.co.uk/screw_on_sprockets/trialtech_sport_screw_on/c73p10409.html

Strange that the sprocket's too narrow. I've only ever seen this on cheap shit sprockets. Dura-ace sprocket's aren't that expensive.

BB spacers also fit

edit: ^^ bah I was doing some research, then started drooling over Di2

Sprocket is actually quite nice, think it's pretty old, doesn't feel cheap. Think it may be more the cheap quando hub it's going onto. Think the inadequacy is between 2-3mm so going to plump fopr a 2.5 mm spacer and go from there.

Just to be sure, would one of these be suitable?

http://www.chainreactioncycles.com/Models.aspx?ModelID=36416

Thanks.