To seal the decal I sprayed the fin with Tamiya X-22 Clear and when dry I VERY carefully masked the white outline with Tamiya masking tape and used paper to cover the 'NF' code letters and the five chevrons before taping over the top of them;
I mixed a near-as-dammit match of Tamiya XF-7 and white with a couple of drops of X-22 to give it some shine and sprayed the fin. After this I VERY carefully removed the tape as it could have easily ripped the decal and at this point things were either going to go very well or very badly (and usually for me it's the latter).
Therefore I'm pleased to say that it worked! I did rip a small part of the white trim but touched it in with Vallejo paint. YAY for me!!
Maybe next time I won't pick such complicated colour schemes :o)