I'm sure that your land is classed as private land. Even if the council owns it, there is no public access. As you are in Scotland, Black v Carmichael 1992 SCCR 709 comes into effect. As you probably know this renders clamping on private ground in Scotland illegal as it is said to be Theft and Extortion. Both of which are criminal offences. You could see if the Police would be interested in helping.
Personally, I would not even entertain the idea of negotiating with them, they are blatantly breaking the law. Also complain to the SIA about their actions.
sounds awesome, thats the green light you need, go download a video of the boxing fight between Black v carmichael, get a long extension lead and move your telly outside, if whilst grinding the clamps off the clampers come back just show them the fight and they will be scared and will walk away in fear.