Chris, Hannant's used to stock the 'Tally Ho' range - I got some from them, along with red serials and Dull Red codes. But I believe Tally Ho have stopped production - not sure if they have stocks left at Hannants.
If not then a call to Hannants might clear up the problem (e-mails seem to go unanswered!), as I think there are some in the Xtradecal range.(you could check the picture of the decal sheets they have, to see the style).
If all else fails, then print some onto clear decal sheet - makes it easier to apply too, rather than having to cut and apply separate letters and numbers.
EDIT:- Forgot to add, the WW2 serial numbers were 8 inch, so the 4,8,12 inch is an identifier that they're post war. Look for 8 inch serial numbers.