My 1.6Petrol P-Reg Astra wont start, please help?

astra 1.6petrol (97-preg) wont start as if battery not connected. have replaced battery & worked for 5 minutes then replaced alternator & worked for another 5 minutes, it just cuts out, buttery light comes on & wont start again. now its completely drained as if no power at all, checked all wires, fuses & connections but cant see anything wrong, any help gratefully received, thanks in advance! - (JHoltWeaver@aol.com)