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Edited 27 November, 201510 yr by Scott1874

When you started the car in 1st gear I am assuming you had your foot on the clutch? If you did and the car moved forward then the clutch is not clearing. Did the pedal feel ok? Is the puddle on the ground DOT 4 fluid from the clutch slave cylinder. Try rocking the car  forward and backwards to see if the gear lever will return to the neutral position.

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Edited 27 November, 201510 yr by Scott1874

It sounds like the clutch slave cylinder could be the problem. 

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Edited 27 November, 201510 yr by Scott1874

What about the gearbox stuck and the leak though? How much would a garage roughly charge for the slave clyinder?

I don't think that the problem is with the gearbox. Do you have breakdown cover that will come to your home? If so they might be able to diagnose the fault.

This sounds much more like a clutch hydraulics issue than a gearbox issue, not least because if it was gear oil you could get 2l of it, and it would stink to high heaven.