2001 clio 1.6 rsi, intermittent starting

I have a 2001 clio 1.6 RSI and have been experiencing intermittent starting issues.

Sometimes the car starts fine, however on occasion struggles as if the battery is flat and eventually starts. However sometimes it does not start at all, making a sound of turning over every 5-10 seconds but the engine does not fire. After connecting with jump leads to another vehicle it starts perfectly everytime. I have recently had a new battery fitted as a mechanic said that was the cause but it did not solve the problem.

I have read around the forums and spoken to people and have heard a number of possible causes such as a TDC sensor or the altenator. If anyone knows what the probable cause could be, and can offer help to a solution, their help would be greatly appreciated,

thanks in advance