Alex, I decided to go round and check I can get all of my bolts undone, before I start the body lift on mine. I already knew the rear o/s one was seized, so I drilled a small hole in the side of the mounting block, so I could get to where the captive nut is, and sprayed it with loads (and I do mean loads) of the Cheap Lidl's rust dissolving penetrating oil.
I did this about a week ago, and when I went out and tried it today, it undid nice and easily. I tried the rest of the bolts, and the middle ones are also solid, and feel like they might shear, so today, I have taken the front seats out, lifted the carpet, and drilled a hole over the location of the middle bolts, where the captive nuts are, and again, filled it with Lidl's stuff. I will leave it a few days, and try again.
I think it may be worth you doing the same, that way, when it does come to time to do the lift, it will be a lot easier, and mean there is far less chance of something going wrong on the day, and leaving you stuck without a car.