Lynne, the key isn't that special. You can buy them readily from electrical suppliers; next time you're in Leicester ask for a universal cabinet key- they cost about seven or eight quid for a posh one. Or get one here, a bit pricey (plus VAT plus postage). In extremis PM EmmaB, who knows me, and I'll sort one out for you. But case the house first to make sure the box IS outside.

If it's just revenge you want, get hold of a length of the transparent tube that's used for home brewing etc, and a can of expanding foam. Attach the tube to the dispenser tube on the can, stick it down his drain so it goes well round the bend, and squirt well. Do the job properly and it will cost him thousands by the time they've located the blockage!