Hi Serena,
I've always used the sill jacking points, that's what's recommended.
Sometimes experimented with parts of the rear suspension, i.e. where the arms join the chassis/body, but I don't honestly think I'd lie under the car in that situation. The problem with any BG now is the structure will be weak with rust, so jack it under something that's solid!
Do you need to remove the wheels? An option (if you don't) is to use ramps. I've recently changed the old metal-rung ramps for a plastic set, and despite initially baulking at the word "plastic", they are brilliant.
I'm guessing you're welding the rear sills, though, so you'd need a hefty asbestos/heat-proof board between the sill and the tyre!
