2008 Vauxhall Corsa loss of power, rough idle, burning smell

Just bought this car from Arnold Clark last week, they fitted a new sump gasket as apparently there was a leak. Car drove ok for first day and a half but now has a burning smell after driving. When idling its pretty shaky. The power loss is awful, can be driving along at 40mph and suddenly speed drops to 20mph. Pulling away from junctions and roundabouts is a nightmare, no acceleration, car just trundles along then finally kicks into life and 2nd gear seems to be non existant. Drives better at 40mph in 3rd gear than it does in 4th. They're taking it back in to have a look at it on Thursday but seeing as how they are complete cowboys I wanted to have a rough idea of what they should be looking for/fixing.