Doesn't it have a timer?

Most electric ovens, in my experience, have timers that you can set and on my mum's you can set the time it goes off as well. She does it all the time and has never had a problem (actually, she did once when there was a power cut but it didn't catch fire or anything, just didn't come on at all).

You'd have the oven on for up to 3 hours if you were in the house I'd imagine (roasting stuff, slow cooking things) so if you trust the oven to do that I'm sure it'll be ok if you are out.