I've registered my C5 with immobilise, so if it is nicked I can get it back, but I've not considered any of their various marking kits. I don't know if the C5 has anywhere suitable for the bike tags to go, but perhaps some of their branded, numbered stickers would be a good idea?
I remember getting my bike security marked a while ago, I considered the tag at the time, but I discovered that few police stations outside of London have the reader for the tags, and it's hit-or-miss if it gets checked with the reader anyway (if the sticker has been removed, this is). Of course, this was a while ago, so things may have changed now…
Edit: Or perhaps one could epoxy one of these tags somewhere to the body?
https://www.immobilise.com/view.php?stage=product&category=1&product=3