This must have been asked 100 times before though not much comes up in google groups. I assume from what's there you can do it simply by putting an imperial olive on the pipe.
Next question: where to buy imperial olives...I haven't looked very hard yet, but screwfix, for example, don't sell them.
M.
Any plumbers merchants will do, local Yelow Pages and you'll be sorted in 10 minutes.
B&Q warehouses stock these, as do any proper plumber's merchants
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