Check this thread: http://www.daihatsu-drivers.co.uk/node/748

The thread explains briefly how to adjust the clutch cable, verify this is not the problem first.

If you still think it is an oil leak, then my logic would suggest it can only be the oil seal on the input drive of the gearbox or the oil seal on the back end of the engine crank shaft; an oil leak will only effect the clutch if it results in oil getting on the clutch and these are the only oil seals in the bell-housing where the clutch is.

Assuming adjustment is fine, then I would think either the clutch has worn out, or it is slipping because oil is getting on it. Either way, the clutch is going to need replacing and this will mean the engine has to come out. Because of the time and effort involved in getting an engine out, I always replace the clutch-plate, pressure-plate and thrust bearing together and if there is any evidence of oil leaks, identify if it's the gearbox or engine side and replace the relevant seal (You might even consider doing both; neither are going to be much work compared to removing the engine).

Duncan

warning: may contain traces of nut.