My Renault Megane has an intermittent starting fault. ?

It has been with the local Renault dealer for some four weeks and still they can't find out why it starts and then won't start again after some 15 minutes or so. To date they have changed the ECU and found it to be serviceable, and they have gone through 'their protocol'! After alll this time they now think it may be the tappits! Any ideas?