Crap qu I'm afraid..but why does my fridge have such bad condensation that there are often puddles running down the back wall?

[Slouchy]

?

Are the coils covered at the top , adequate ventilation ?

Door seals may be leaking or the drainage hole at the back is blocked.

Isn't that how they are menat to work? The ice melts and runs down the back wall and into a unit at the back whick lets it evaporate?

Like SG says, check that it's not blocked where it drains. Have a look at the back and see if it's evaporating ok.