Less than £50 won't really get you anywhere.
I'd personally start with a new headunit, it can be surprising how well stock speakers respond to better amplification and then you can always move on to upgrading speakers later.
If you're not fussed about features like SD card support, iPod functionality, bluetooth etc. then something like this
Pioneer DEH-1300MP would be a good start (shop around though, it may be cheaper elsewhere when you account for delivery). It has Pioneer's 4x50W MOSFET amplification which may well make a good improvement in your sound quality without touching the speakers.
Now with a 2002 Corsa, you've got one of those stupid massive stereos that's an odd shape to start with too, so you'll need some sort of faceplate adaptor too. You'd need to find a fascia adaptor and I suspect a wiring adaptor too, for a Corsa C, which will probably set you back another small lump, just so you can fit the headunit at all. Something like
this and
this are what you'd want, though someone else here may know better.
edit - Oh you may also need some removal keys if you don't have any or know anyone who does, otherwise you may struggle to get the existing headunit out.