Just for reference, in case anyone else comes across one of these in the bottom of their toolbox, I deduced that this is the pin layout and as Big Herb says, its a changeover relay:
30 to 87a has continuity when relay unpowered, 30 to 87 has continuity when relay is powered. Pins 85 & 86 are the trigger.
Oddly, although the relay had the number 30 written on it (see below), it seems that this had little bearing on where pin 30 was!
Apparently these relays are common on Nissans and other cars made in the far east as well as Vauxhall. Make is 'JIDECO' (now merged with another company) although Hitachi also manufacture them. They appear to come in 'green' or 'blue' so presumably one is higher amperage. The Jideco Relay 60419V (1T 8-94235-627-0 MR5A-211-112V) in a blue case is rated at 30amps. Either way it'll handle the OOOHGAA horn no prob although nowhere meaty enough to handle a split charge system.
The only unanswered question in this puzzle is how I came to have the thing in the first place....
More here
http://www.e-jbc.biz/relay-jis-mr5a-eng.html" onclick="window.open(this.href);return false;