Either I've lost one or someone's had one away. Anyway, in need of an off side rear reflector.

Surely there must be somewhere where a small piece of plastic can be bought for significantly less than £20?

Isn't there?  Series 2A 1961, 1975-78, made to last forever.
Series 3 Lightweight, 1973, 1999 - 2012, still regret selling this car.
Freelander 1, 2005, 2005 - 2008, sold the minute its warranty expired.
Defender 90 200 TDi, 1985, 2012 - 2014, paid far too much for it.
Defender Wolf, 1998, 300 TDi, 2017 - 2019, too expensive to maintain and the Army hadn't.
Defender Puma, 2009, 2014 - 2014, not a sensible family car.
Freelander 2, 2012, current, is this the car to last?