Vauxhall won't start at random times

[Johnnyren]

Hi there
I have a Vauxhall mk4 2000 plate , which seems to refuse to start once every month say . The first two times it happened I thought it may just have been a flat battery and I've had it going by either a push start or with jump leads . It happened again today a month or so since the last time , this morning had had drove it thirty miles or so to work , popped into a shop for a paper and when I came out it was trying to turn over but wouldn't start.i managed to get a push start and carried on the last mile or so to my work , When I tried to start it earlier to go home it started no problem . Any ideas what it could be ?