It looks like this; it's about 8mm from one point to the opposite flat edge:
Should I have got an appropriate tool with the bike when I bought it (3 weeks ago)?
Edit:
It looks like it may be a Trans-x skewer having looked at some pics of other ones. It's a bit cheeky that this wasn't mentioned when I bought the bike; I had assumed it was a standard allen key jobby; good job I didn't need to change an tube in an emergency.
I'll pop to the LBS and see what they say.