2 of these mounted under the beak are cheaper and much, much louder than a BMW one:
https://www.ebay.co.uk/itm/Fiamm-92...5?hash=item2ad478569b:g:2dQAAOSwpG5de3vW#rwid
This pair is even better, even louder at 128db and more expensive:
https://www.amazon.co.uk/FIAMM-7410...=1&keywords=fiamm+horns&qid=1594000987&sr=8-9
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Thanks for the links Bobad, but I'm after the BMW 5 series with the same plug as the OE one.