it appears do be a 8-pointed star. Does the star project, or is it a socket? does the centre of the star push in?
the common star key for mortice bolts only has six points.
Look at all the other window latches in the house and their keys. there may be a trade name.
there are several companies that specalise in replacement keys. For example
Replacement Keys Ltd stock a range of 168 different window keys. We use the latest key cutting technologies to fulfil 85% of customer orders within 24 hours.
www.replacementkeys.co.uk