Well, I spent some time looking closer myself at the garage today and am sure now that the small hose is a bodge from a previous leak.
It goes from a proper spigot on what might be the thermostat housing? Not completely sure about that but have attached a photo of it which shows an electrical connector immediately above it.
The other end attached to a short metal jointing piece which is connected to a hose running under the supercharger.
So i do think its a bodge and that small hose I've been searciing for, with your help, doesn't exist as a Jag part after all!
If that all makes sense to you I might try a better fix than what has failed this time.
Thanks.
Phil.