vw polo speed sensor probs

i have a 2001 1.4 16v polo . the ecu light came on so i took it to independant vw garage and they tested it and came back faulty speed sensor . they changed it and deleted fault and car ran perfect on test drive . but when i picked it up there was a clicking noise from dash and ecu light , oil light came on and clocks stop workin. could this be a timing error or bad cambelt or does anyone have any ideas as when retested by garage keeps coming back faulty speed sensor , but we have changed it , thanks john