Thanks everyone. So my learning from this is that it seems to be easier to create the codes using a reverse process, i.e. I should have painted the white first, before any camo colours were applied, then masked the white with tape of consistent width to form the letters. I did my Mk II Spitfire DW-O that way and it turned out better I think. Mind you, these letters were much smaller and tighter so any slight variance in width of the mask became very noticeable. Anyway, I'm satisfied with the results but am not sure this would stand up to model contest scrutiny.
Having now finished the codes, I did some subtle panel line highlighting with a sharp .5mm pencil, darkened the engine area underneath with pastel, and sprayed a coat of Future on making sure there was a good thick coat over the codes. I'll let his set for a few days, give it a light sanding, and then get a flat coat on. Here's how it looks after today's work. The prop has been stuck in temporarily.
Thanks again everyone. I appreciate your comments.