Giles, looks like a trip to Halfords may help. These say they cover 90% of cars and h4 h7 h1 bulbs. Sods law means your Dad's will be in the 10%, but worth a shot for £1.80 without club discount!!
https://www.halfords.com/motoring/car-parts/electrical-lighting/headlights/halfords-headlamp-retaining-spring-h4-hfx694
Alastair