By the way how important is the 'box'.. Only £120 odd for a wooden carpet covered box seems steep. My dads got lots of wood tools and I'm sure he could knock up a box easy enough (or I could).
Depends. If the sub is a bass reflex (roughly, has a port) then matching the sub to the box is important, and that makes building the box sufficiently accurately a little more critical. For a sealed box, things are easier in the sense that if the dimensions are a little approximate, it doesn't matter that much (it changes how deep the bass goes and some other parameters slightly, but not dramatically). On the other hand, needing to be sealed is a difficulty, because approximate construction probably won't lead to good sealing and maybe the panels will flap around a bit, and that won't help the sound. (And, any filling you apply in the box will change the effective size slightly, and that would be a problem for a box that has to be built accurately to a size because it has to be built accurately to a size that isn't the size that it is. Which is a complication.)
In any case, it depends on what exactly you want and how critical you are. If you'll be happy provided that you get a bit more of a rumbling noise at low frequencies, then that's relatively easy. If you are bothered about how loud it goes, the exact matching between the sub and the existing speakers how well it matches with the room response of the car and whether the impulse response is still clean, you are making things more difficult, and if you are bothered about all of the above, then things have got really difficult, and you probably won't satisfy all of the above simultaneously with your skinflint ambitions, however hard you try.
Also, I don't understand how the 'Watt' thing works if you go for a separate sub and amp..
Watts is volts times amps. You need enough. All else being equal, which it rarely is, more is louder. If you turn it up too far, you can destroy the speaker through over-extension. If the amp won't deliver enough and you turn it up too far, it can still destroy the speaker, but without the over-extension.
Also *tightwad alert* is there cheap subs, cheap amps, etc that I could use?
Probably. Amazon tends to be a good place to look. The bay of fleas, I don't tend to trust, but if someone recommends someone trustworthy, that might work.
Matching the drive unit to the box is an issue, so if you get the box and drive unit together, that ought to (note: ought) be a help. Otherwise, you should be looking at the Thiel-Small parameters, in the first instance, and I don't think that you want to do that.