2000 Volkswagen Golf intermittent not starting!!

Hi, I have a VW Golf 1.9 Tdi which is fine most of the time. Every so often for no apparent reason it won't start. I turn the key and it will try but wont start, sometimes if I try again straight away it wil be fine, others if i leave it a bit then try it wil start, and sometimes it just wont. Recently my dad put it in gear and tried to start it without the clutch so it would jump,after that it then started!? Tried that today however and it had none of it! There is no apparent logic to it at all. I have had it at the garage and they thought it was solenoid but doesnt seem so. I called the AA out before and then felt like a right idiot cos as soon as the guy turned up and I turned the key it started no bother!!? No-one seems to have any idea what it is and im fed up now as I never know when it is going to do it! Please has anyone had a similar problem?!