Anyone know a source of the same thing for plastic conduit please? (I
know TLC do reducers but they seem just to join 25mm to 20mm conduit
whereas I'd much rather just have something to lock 25mm into a 20mm
hole.)

www.farnell.co.uk

Reducer (M20 fitting into M25 hole, eg, CW20 into M25)...
- 1204224 - Chromed Brass - £1.31
- 1204216 - Plastic - £1.39

Enlarger (M25 fitting into M20 hole, eg, CW25 into M20)...
- 1204229 - Chromed Brass Round - £1.90
- 4380460 - Brass ATEX Hex - £5.61

M25 into M20 in plastic may be more difficult, do a google under
"Enlarger" or "Adapter" rather than "Reducer".
--
Jason.