Just picked up a couple of personal alarms from Poundland. They are the size of a box of matches but about half as thick. They make a loud noise (130db) when the cord is pulled. You can shut them up again by pushing the pin back in. The supplied batteries are alleged to last for 1 hour and they can be replaced.
They are made by Signalex.
Can't find a picture but someone is selling them on Ebay if you want to look it up.
If this tickles your fancy, give me a Rep will you, I have been stick on 984 for a long while!