Have you checked your user manual as there are normally loads of conditions that have to be just perfect for stop start to work such as outside temperature, air con on/off, battery charge level etc etc
Hi Daffyd welcome to the club. Have you tried Euro Car Parts? http://www.eurocarparts.com/ecp/c/Nissan_Almera_1.6_1995/p/car-parts/car-service-parts/major-service/distributor-cap/?404700121&1&2beab0b8093963f7139f6471517adf868cc145de&000104