my astra 07 did that a couple of times, topped up the coolant and it never happened again. so that's my advice, top up coolant, with the lately hotter weather, your car will be doing more to cool down therefore using more coolant.
so top that up, don't just use water, buy actual coolant to do the job properly for a few quid, and run it for a couple of days, if it still happens get back to us.
might as well try the cheaper options first before you pay a mechanic £xx just to even look at it!!