Sounds like your clutch is slipping. Could be due to high mileage, oil contamination (leak from g/box or engine) driving style of driver(s) e.g. holding car on hill for some time instead of using foot/handbrake; riding the clutch i.e using as a footrest. A friend of mine just had an Astra estate done, and was quoted about �400 by Vauxhall & Mr Clutch - apparently on her model they have to drop the front subframe! Let`s hope yours is more conventional to repair, but you will want to know what caused it to go.