Hi Marge,
Pete won't mind me butting in, Sidney will have a death certificate, probably within the GRO WAR Deaths, Army, Overseas deaths 1939-1948'. The GRO is the place to start, they should be able to help you and produce the copied certificate.
It's horrible being passed from one organisation to another without too much consideration for your feelings. The certificate may even shed some light on the cause of death?
Steve.