i have a few questions about changing pickups.

1) is it worth about £100+ to put EMG's into a mexican fender jazz

2) how much difference does it make to the tone?

and 3) is it easy to change them yourself or if you take it in to a decent shop will they do it for you?
well, the really big thing about emg's is that you get rid of most of the noise, becuse your output is hotter.
you can get into problems with your input stages clipping, which is what they go for in guitars, but for a bass you might end up with distortion before you want it.

as for the sound, they're known for being well balanced, that being said, they dont add much 'color' to your sound, so you have to tweak the eq to make it interesting.

and you can put them in yourself, but they're a little harder to do than passive pickups, mainly because you have preamps and batteries to deal with. and you might have to rout a cavity out to fit the extra electronics or if you wanted a battery box. a shop would do all that for you.

i would have to recommend getting a shop to do it, just because they're a hassle, and you want it done right, not that you cant do it, its just that the time it takes is worth paying for it. and when its done it'll sound better than it did, so really, its worth the price, and sometimes you can buy them through a shop and they'll put them in for free (you might have to ask)
if your bass has passive pickups as standard does it need a load of routing and shizzle to fit the preamp or is it just not possible?
well, sometimes there isnt any routing that needs to be done, but there is a bit more electronics to be fitted, and i believe you need new pots because of the difference in output and what the emg folks want, they come in the kits if you go for one of those.