Now we are supposed to be going to the Green Man Festival in Wales tomorrow (100 miles or so) and today the clutch is slipping in 3rd and 4th

Since changing the gear oil there has been a black (engine?) oil patch on the ground in the morning after driving the day before, which looks like it has come from the front of the engine. There were always a couple of drips from the back but this is definitely a new leak, maybe it's linked to putting in fresh oil somehow if the old stuff was full of gloop.
So two questions if anyone can help: Is the oil leak most likely a sign that the crankshaft seal has gone and the clutch slip is a result? Is there any chance I'm going to find somewhere which can replace the clutch and oil seal tomorrow morning in the West London area?? Or somewhere on the way to Wales...
Thanks for your brain power ladies and gents, and advice gratefully received, bit stressed out here.
Don