I vote for the bottle jack too. You can get a small 6 Tonne one for £15 or so from Machine Mart, if you pop a couple of 12" bits of 2x4 wood, you can do almost anything!! The wood helps build up a bit of height when needed and will spread the load when jacking on soft-ish ground. They are also dead easy to use.
Oh, I would also get a pair of axle stands if you are doing anything other than changing the wheel. Most jacks are quite unstable (except trolley jacks) and hydraulic ones can fail. The consequences of skimping on £20 for a pair of 6 Tonne stands doesnt bear thinking about.
Bottle jacks can be useful for other jobs too, I used one to jack the axle away from the chassis to remove a rear Rangie spring the other day, which got around the lack of a spring compressor.