I've recently worked for two startups, my previous around 65 employees, my current around 10. I was asked this in both interviews and it's even in my contract for both companies:
conform to such hours of work as may from time to time be reasonably required of him on the understanding that the performance of his duties may require the Employee to work outside the Company’s normal business hours of 9.00 am to 5.00 pm. There is no entitlement to be paid extra remuneration for any additional hours worked in excess of basic weekly hours, as this obligation has already been taken into consideration by the Company in determining salary levels;
The key part of this clause is "reasonably required". I work extra hours, log on weekends and holidays etc. when something goes wrong. My salary takes this into consideration that I will do this, but it's usually shared about in the team, depending on who is actually available.
It's also worth noting that, although they don't say it (because then they'd be obliged) if you're the person who stays late, offers to log on when there is a crisis etc. You're going to get a good bonus. They're likely to be more flexible with holidays. They are going to be ok with you clocking off early to catch a train, or see your daughters play.
I can't say that this is the case in every company, some will take the piss, but if you think there is a good atmosphere in the office and like the company, it's unlikely you'll get one of the bad ones.
p.s. Also UK (London)