Several years ago a got a gas heater from 4Seasons, it has an open flame like this. 

That summer we didn't use it as we had a newborn child and didn't get outside in the evening much, the next year it was too wet, year after I went away and now this summer I've dusted it off.

It was used the day we got it and never again.

When you start it I get the igniter clicking like the gas hobs in the kitchen but it doesn't light. Any tips on how to work out why?