Many thanks Dave. To answer my own question, I dug around a bit and found the following as suitable for the 1.4 AUA engine, taken from the respective manufacturers websites:
NGK
Stock No.2397. Exact item code BKUR6ET-10 (Thanks Dave!): 3 Prong. Preset gap.
NGK list an 'upgrade' plug as:
Stock No.3764. Exact item code BKR6EIX-11: 1 Prong. Iridium centre electrode. There are lots of user reviews for these on Amazon, mostly very positive.
BOSCH
0 242 232 802 (FR 78 X) spark plug set: 4 Prong; Threaded length 19mm. CrNi electrode. Tightening torque 28Nm.
0 242 232 502 (FR 78 X): Individual plug. Details as above item.
0 242 235 947 (FR 7 LDC+) spark plug set: 2 Prong; 0.9mm spark gap. NiY electrode. Tightening torque 28Nm.
0 242 235 668 (FR 7 LDC+): Individual plug. Details as above item.
Bosch list both of these plugs as suitable. No preference or product hierarchy is given.
BERU
Z90 (14 FGH-7 DTURX): 3 Prong. 1.0mm spark gap, preset. Threaded length 19mm. Tightening torque 25Nm.
UXT1 (Ultra X Titan): 4 Prong. Threaded length 19mm. Tightening torque 25Nm.
UXF79P (Ultra X Platinum). 4 Prong. Threaded length 19mm. Tightening torque 25Nm.
CHAMPION
CET2: 1 Prong. 0.9mm spark gap. Threaded length 19mm.
OE032/R04: 3 Prong. Preset spark gap. Threaded length 19mm.