Lots of posibilities. Tickover too low, Combustion out of balance - plugs dirty / fault, low voltage ignition fault, HT vo;tage ignition fault, anti vibration mount fault, engine fault - crankshaft, bearings, pistons, gearbox fault, petrol supply fault, etc. The list goes on. Could be cheap or very expensive to fix. Suggest you be wary, particularly if it is a private sale. Is there a service record?? This can be obtained from a Seat dealer, if it has been serviced by Seat.