The speed signal comes from a sensor mounted in the back of the speedo itself on early models. These are prone to failing.
Later models without mechanical speedo cables, the speed signal is derived from the ABS. Either the rear right or the front left wheel sensor. If this signal is faulty the speedo itself won't work either.
thats what i asked the vx chap lee - he then went on to say its all cables from the gearbox to the speedo, dash comes from 96 turbo - think i may sod it all off, goin over my head now - thanks lee - that explained it in one answer