>Does anyone happen to know of a supplier (ideally UK-based) of
>small (3-inch), round, high-impedance (25 ohms) loudspeakers?

The nearest I can think of is a 100 ohm speaker used in model railways locos 
for sound.

See http://www.esu.eu/en/products/accessories/loudspeaker/ for 
possibilities.

The (official) UK distributor is Southwest Digital 
http://www.southwestdigital.co.uk/ and they should be able to get you any of 
these.

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