Finally got around sorting out the horns, discovered that they are the same ones fitted to ford transits / connect / mondeo, all at various prices. Whilst deliberating as to whom I should buy them from, I came across a supplier on Ebay, who sells a universal pair with the appropriate connector,  ordered them they arrived quickly, so I set about fitting them, it was a doddle, I removed the front grill, slipped a 10mm rachet spanner in behind the bumper, spun the nut off, unclipped the wiring harness, dropped the horn assembly out, mounted the new horns on the old bracket popped them back in. I now have decent loud horns, instead of one's that sound like a castrated rooster, And it took less than an hour!  

Once I had the old horns out, I removed the wiring harness from them, and found one of the terminals to be rusted, so only one was working. 

Still, twenty quid and roughly an hours work was well worth it. 

 

Handy to know about the parts-bin raiding that went on - the screenwash pump on many early S Types was also shared with many Ford vehicles and a Volvo too!