If you tested it with a single filmpack, it may be a battery fault since the shutter is battery-operated, so trying with another one might worth the the shot you'll waste...
120£ is really close to what I paid one year ago for a near mint 680 from a british guy on the bay...
You'll better try to fix it yourself... there are many useful guides for the very similar sx-70 on the net, most likely problems are dirt on shutter blades or sticky seals in mirror return.
Do you actually see ANY image in the finder, even a very dim one? try to point it to a very bright scene or a light sources, if you see a dim image it will be something related to shutter blades (grease, dirt, or a faulty battery), if you see absolutely nothing it is probably something withing the mirror return, where aged light seals might become sticky and prevent it to return in position. I think there is a mechanical latch that also prevent the shutter opening in this case.
Also, do you hear anything when half-pressing the release button? does the camera try to focus or stay silent?