Passat 2005 diesel 1900 TDI Highline, just over 100k miles. Refuses to start - but only sometimes! Recently it went days without starting then when the garage comes to tow it in for repair - it starts!

Symptoms - engine turns over ok but doesn't even try to fire, the failure to start is not related to engine temp, sometimes it refuses to start from cold sometimes refuses to start when warm. When the engine is running, there are absolutely no problems.

Not sure if this problem is related to last year when it was filled with petrol (and driven). The garage felt it probably wasn't, and yesterday they ran diagnostic which didn't reveal anything, they suggested that it might be the temperature sender switch so it was replaced, changed fuel filter added fuel treatment - that was yesterday. This morning, ran my wife to work - no probs, but just now when I went to leave - refuses to start!

Any ideas?

Biffo