Well Alex, it's all set up so I thought I'd let you know how I got on.
We watched Batman Begins, and the Dark Knight to test it out. Put simply, it's transformed our movie watching experience! OK, it might not hit the heights of many other home cinema setups, but it's a good sound that for the first time has immersed us in the atmosphere of the movies.
Before: Rotel RA-05 - B&W 685 (stereo) --- it sounded good, great quality, but only stereo.
After: Onkyo HTX-22HDX & SKS22X --- wrapped around in sound, a much better movie experience.
Even with this being a relatively cheap way of achieving 5.1, it has completely changed how we experience a film, and I certainly wouldn't want to go back to watching/listening to movies in stereo. I think I've still probably got a little tweaking to do with individual speaker levels, but I'm very happy with it, it was very easy to set up.
Alex, I know you said you are thinking of just getting 2.1 to start with, but if you are getting it with primarily films in mind, (I wouldn't listen to music through it), then I'm sure the extra speaker set is worth another £80.
All the best!