Thanks for the replies. After asking in the local plumbing supplies
store a bit of goggling, this sorted it - no silicone, no plumber's
mate, no PTFE tape needed. I've seen it described as a "basin mate" and
a "bowl sealer".

http://www.ebay.co.uk/itm/231601813077

It's a rubber wotsit with internal ribs/threads and a conical shape.
You push it onto the bottom of the waste and the conical bit forms the
seal with the bottom hole in the basin. Tightening the securing nut
compresses it, sealing the threads. As a bonus, it centres the plughole
in the top of the basin.

2 quid from BnQ (plumber's store had no stock) and worked a treat.

I've always felt it should be possible to get a good seal without
bodging with silicone or plumber's mate, especially if the surfaces are
clean. A bit of washing up liquid for lubricant, that's all.

This guy uses one:

https://www.youtube.com/watch?v=jaPsf2xTZfc

but also uses silicone, which is cheating IMO

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(='.'=) Bunny says: Windows 10? Nein danke!
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