If you're jacking a car up to do anything other than changing a wheel you absolutley HAVE to put the car on stands! A trolley jack, let alone the cruddy little scissor jack in the boot it nowhere near stable enough to leave a car on while you crawl under it
Stands are reasonably cheap, you should be able to score a set from superjunk autos with out violating your wallet. A set of 4 is all you'll ever need, but if budget won't allow, then 2 will suffice.
On my LR I used to jack the front up one side at a time using the end of what looks like the end of a chasis rail as a jack point and then place the stand as close to the jack as possible. Just make sure where you position the jack and the stand is a strong reinforced part of the floor pan otherwise you'll end up with an auxillary floor vent.
The back was a little more tricky. I couldn't find a decent place to put the stands on the floor pan so I ended up jacking it from the centre of the rear suspension subframe (had a handy little dot of paint in the centre of it, why? Buggered if I know) and then placed the stands at either end of the subframe as far out as I could. They're not as far apart as I would have liked but it's stable.
Someone else might have a better technique than me. It's not the easiest car to lift. But then again I'm used to rally cars where you stick the jack under the diff and then the sump guard
Hope that helps, jacking a car properly is really really important as to not damage the car nor end up pinned beneath it. The order of importance between those two will differ between people but they're both pretty important