my mk4 astra t-reg won't start?

my astra mk4 t-reg (club) 1.6 will not start, there is no click when the key is turned. the alternater is new and the central locking and all dash lights work! it did it once before and the mechanic claims to have replaced a 'relay' and it was fine for 2-3 weeks.....but now we're back to square one! could it be the ignition electrics or indeed a relay again??? HELP!!!!!!