what to do when the inside of the tap won\\\'t unscrew

I have a dual mixer tap in my kitchen, extremely old and limescale-ridden. The cold water tap suddenly wouldn\\\'t close all the way and water kept gushing out. Took the tap cover off to try to unscrew the middle bit so as to replace the washer and/or see what the problem was. Problem is it won\\\'t screw off. tried every kind of spanner or wrench I have, and it is absolutely stuck on there. Tried WD40. No result. Is there anything I can do or any special tool I can try to get it undone so I do\\\'t have to get a plumber or handyman to replace the tap fixture? Thanks for any advice.

Heat would help but you risk damaging things. Best spanner is a close-fitting ring type - maybe a long socket (flank drive would be absolute best but they're a bit exotic).

Trouble with stillsons etc is that they distort the brass so it won't turn.