Take the car back and explain that it was OK when it went in and it wasn't when it came back. Any responsible garage will sort you out. If they don't just tell them you will take your planned £7k complete engine upgrade somewhere else. http://www.ltregister.f9.co.uk/graphics/wink1.gif border|EQU| 0 align|EQU| absmiddle >
Alternatively check out the Vehicle Speed Sensor is connected correctly. Its located directly behind the engine sort of in line with the right hand rocker cover end (Thermostat/Secondary Air Valve) way down on top of the gearbox. You can't really miss it as its the only thing on the gearbox housing with wires coming out of it. Make sure the connector is firmly in place. Trace the cable back and see if you can see any damage to it if the Sensor is OK.
By the way what work were you having done as it may help to diagnose the problem (did the touch behind the dash at all?).