Any question answered...

OK so a mate needs some dirty cheap slicks for 26" wheels (about a 23mm wide mtb rim), found these on wiggle... but they are described as 'imperial' sizing. Last time I checked 26" was 26" but then seen as 700c is actually a smaller diameter rim than 630c (27") then I'm not counting on them being 'the job'.

http://www.wiggle.co.uk/schwalbe-marathon-kevlar-old-tread-mtb-tyres/

Please be aware that tyres marked with a fraction in the sizing is generally classified as "Imperial" (26 x 1 1/2"), whilst tyres marked with a decimal in the sizing is classified as a "Decimal - standard nowadays" (26 x 1.5). Imperial tyres are not the same size as Decimal tyres (roughly 1 inch difference) so please be aware of this when choosing your selection.

and the wiggle item in question calls itself

Black26 x 1 3 / 8" Imperial

Sheldon made too many words, I understand not this imperial vs. imperial sizing malarky.

is 23mm inside or outside diameter? You probably need 26 x 1.5 ( or 1.75 or 2") - between 32 and 40mm