This is my post here so please be friendly, really hope someone can help

I have a V plate 406 HDi that doesn't get much use any more, it's been sat in a garage since January which is when the MOT ran out.
Up until Christmas it was running fine, but having put it away for a bit I've returned to both rear wheels not turning.
The handbrake still pulls with tension, but is a fair bit more "tight" than it was previously, I managed to free the Drivers side wheel by rocking the car back and forth but the passenger side won't budge.
I've read the How-To's on changing the rear drums etc, but I'm hoping it's something more simple than this.
I can see what looks like an 8mm nut where the cable splits to each wheel under the car, but it looks rusty and looks like I'd break it if I tried to move it.
What's the easiest way to free the wheels? I'm more interested in moving the car than sorting the problem, although ideally I'd obviously love to do both

I've read somewhere about an adjuster inside the car, I'm going out for a look now but I'm hoping someone will know where my fault lies

Thanking you in advance !