Hesitant Start

[Philip W 1982]

My engine is mint it's hardly done any miles (03 1.8 LX - 28k) every so often when the car has been running and then I switch off after going to the shops say, then come back maybe 10 mins later and try to start the car turns over like normal but won't start. I have read somehwere that it maybe an air pressure valve??? It doesn't do it all the time maybe 3 times per week. When it does do it I just stop give it a second turn it over and it starts instantly - literally bang on as soon as the key is turned. Any idea what this maybe?? I have totally slavered over the cleanliness of this car it's ridiculous and I do all the work myself and haven't had to do anything other than servicing it yet - touch wood. I was just wondering if there is anything on the horizon that needs sorting?? It only had 11k miles on it in 2011!!