By mains tap do you mean the a outside stopcock or a inside stopcock or the kitchen tap all are on the mains.
If its a stopcock then it will be open and should only be turned clockwise.
You could make it a little easier by unscrewing the gland nut.
If it's the inside stopcock and you can turn off the outside stopcock, then it is easier to remove the tap head from the body, then you can use heat or penetrating oil. Is your outside stopcock just for your property.
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