Defender fires but does not start

I have a 2013 Defender with a Ford 2.2 litre engine. When I turn the ignition key the engine fires and then dies after 3 - 5 seconds. This started early July and took around six attempts to start and then it would run normally and start fine for a few days and then do it agsin but take more attempts before running. It has been getting worse and now, after 30 attemps I stopped trying before I killed the battery. The fuel pump is running, (I can hear it) and there is fuel to the engine. I have changed the fuel filter. Any suggestions. Thank you.