h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

is there a way of releasing valve assembly from the main tap on an old tap that has corroded with hard water deposits

Is there a shut off valve before the tap that needs repair?

I have to be off to work (8 time zones away) so I'll tell you what I would do-
Turn off the water supply and soak or spray the inner workings of the tap in what is here called *CLR* (calcium, lime, rust remover), which is basically Glycolic, Sulfarmic and Citric acids- ask at a hardware store there. It will take an hour or more to get all the lime deposits out. This stuff is very corrosive so wear gloves, a mask and goggles if you can and flush thoroughly with lots of water when done. Then you should be able to free up the tap washers to replace them. If not then the entire tap needs replacing.