Renault Clio 1.2 1995 (N) Help please!?

I have a Clio Vaersailles 1.2 1995 (N) and have recently replaced the battery and alternator. The thing died on me yesterday and I had to call out the recovery. Had a zigzag warning light showing on the dash. Mechanic said the alternator belt had worked loose so he tightened it up, jump started the car for me and 100 yards down the road it promptly died again. Ended up with a tow home. This morning I have tightened up all the parts and leads from alternator, recharged the battery but nothing at all. The zigzag light is still there - is this electrical maybe?It's fitted with an immobiliser which seems to be working but when I try to start it up there is nothing at all - the engine doesn't even attempt to turn over. Pretty sure it's not the starter motor. Can anyone help with some advice other than scrapping the thing!
Thanks