Vauxhall Zafira . car wont start when cold or left for a couple hours

Try changing the fuel filter and seals , check for air leaks

Go easy on the Easy Start. It's too low ignition point for continued use. Specially in a Diesel engine which ignites fuel by the heat of compression. You will do damage with long term use.
Check glow plugs are working. If high mileage and black smoke suspect injectors need to me reconditioned. White smoke when trying to start compression problem / Glow plugs problem. Not sure if that is a common rail injection engine,probably is. If so needs Diesel specialist with software to check whole electronic system for fault codes.