OK so trying to create the white outline on serials by hand didn't work out too well.
First attempt with a Mitsubishi pigmented ink white pen resulted in a wavery mess which just showed how age is taking it's toll! I'm sure a few decades ago I'd have free handed such details with ease. Not any more! A good irrigation with water and everything back to stage one.
Next attempt with water colour paint and a very fine brush was better but ultimately not a result I could live with, so, more water to clean everything off again.
Third try: I applied duplicates of the necessary four numbers from the Xtracolor decal sheet to some spare white decal (from a 1/32 Revell Red Arrows Hawk as it happens!) being very careful not to allow any water on the white decal - hence squiggledy angles.
Then very carefully with a very sharp scalpel I cut out the inner, then outer excess to leave a reasonable representation of what I was after.
Sandy