DSG service is basically remove the drain/overflow plug and drain the old oil out, remove and replace the filter, refit the overflow pipe, gravity fill the box with fresh oil until it overflows, run the engine to generate heat in the DSG box until the oil temperature is between 35° and 45° (from memory) on VCDS and any excess oil has overflowed, stop engine and refit the drain plug. This gives you the correct volume of oil in the gearbox.
If the oil is overfilled or underfilled then the gearbox will suffer, and if the filter isn't changed at the required interval then similar result. If VW are willing to do this at £169 then it's gotta be a worthwhile investment.
