Tenancy agreement & double rent

[ddrmum]

Hello, I'm hoping that someone can advise me on this. I have rented a garage from my council for 18yrs and it's in a block of garages next to my house.
They have now stated that they are doubling my weekly rent as my property is freehold. If I was a leaseholder or tenant then the increase doesn't apply. As a single parent, this increase makes my garage not financially viable for me.
I have asked for a copy of my original tenancy agreement from 1998 which they claim not to have, although they acknowledge that it started then. They sent me a generic tenancy from 2005 which they claim to be similar but will not clarify the differences.
I have now had an aggressive letter demanding payment of the extra charges or be served with a notice to quit
They have also stated that there's no way of directly contacting their legal department for clarification around the tenancy which I know it did not relate to property tenure, but unfortunately I cannot find it and that I should send any correspondence to them
I would appreciate any pointers on this as I'm sure it is their responsibility (as well as mine!) to retain a copy or at least provide a generic copy from 1998 - I'm sure I wasn't the only new garage tenant that year!!
Do they have the right to double my rent based on my property tenure??
Any help or questions I should be asking to get clarification would be amazing. TIA x