Kmbo, just looked on NHS Direct website, read this:
Signs of serious head injuries, concussion or damage to the brain include:
Lasting headache that gets worse or is still present over six hours after the injury;
Extreme difficulty in staying awake, or still being sleepy several hours after the injury. It is fine to let children go to sleep after a slight bump to the head, but you should check on them regularly and make sure you are able to wake them.
Nausea and vomiting several hours after the injury;Unconsciousness or coma;
Unequal pupil size;Confusion, feeling lost or dizzy, or difficulty making sense when talking;
Pale yellow fluid or watery blood, coming from the ears or nose (this suggests a skull fracture);
Bleeding from the scalp that cannot be quickly stopped; Not being able to use part of the body, such as weakness in an arm or leg;Difficulty seeing or double vision;Slurred speech; and Having a seizure or fit.
If any of these serious symptoms are present, particularly loss of consciousness (even for a short period of time), you should call an emergency ambulance