I have the above mentioned bike and the drivetrain is completely shot.
On the dole so not much cash, seen a cheap triple chainset on e-bay for £15 but the rear hub is (i think) a screw-on 7-speed cassette. Where best to find a replacement seeing as i can't really afford a new wheel and cassette – i.e. a modern jobbie.

Just search for "7 speed freewheel" on Ebay, plenty for less than a tenner.

sorted! now, i've never replaced one of these before – need any specialist tools buying before i start?