Fiat punto starting problem! Please help?

Hiya I have a mk1 fiat punto,
For the past couple of months it's had trouble starting, it turns over but it doesnt start. It doesn't matter what time of the day or weather. It has started at midnight in frosty weather but not in the middle of the afternoon. It seems te only way to start it now is to bump start it, obviously this is very annoying and difficult for a garage to diagnose as it only seems faulty when it doesn't start and when it's starts its fine.
I recently changed the power steering ecu and motor because that broke.
I've only had the car 6 months tops and it's driving me crazy.
I just don't know what it could be.