Yup - it's loud!
OK, time to update this thread - it's only taken me three months to get here!
What turned out to be the correct part arrived in January and cost about £12 (first photo below). Sure enough this fitted perfectly into the wiring loom, which offers the same connector as the one on the right in the second picture.
Interestingly enough (well, for somebody anyway) the female connector that is part of the OEM horn is not perfectly oval. Instead it has a solid tab taking up about a quarter of one side, kind of what you'd expect to see if you had a female version of the right-hand connector in the second picture. It matters not, part number
80 00 0 610 619 is the one you want.
(Thanks theEnglishman!)
If you take a closer look at the connector on the end of the loom you'll see why this is so much better, in engineering terms, than the cheap and nasty spade connectors you would otherwise be polluting your OEM wiring with - it's got a locking tab on one side and no less than three rows of silicone rubber seals keep all and sundry away from your precious contacts.
Since you have to fit female spade crimps to the yellow leads of your freshly aquired 2 pole connector in order to mate it to the Stebel horn (which, ironically enough, costs less) my advice would be to not trim the yellow wires too much. After all, moistre and corrosion is bound to mess up the spades at some point and you'll thank your foresight for leaving enough slack in the wire to re-do your handiwork in the distant future.
The horn does not seem to be polarity conscious and reversing the spade connectors had no obvious effect on the volume or the tone. If you plan to test this as soon as it's fitted (and who wouldn't?) then do make sure you fit some ear plugs first, eh? It's easily as loud as the horn on a Land Rover Discovery and isn't muffled by bodywork, which is probably the reason you bought it.
Happy Spannering!
Mup.