Had the nc30 race bike out for a wee test run and the clutch cable was really loose so i tightened it up on the adjusted and it was fine.

Then today we were racing and in the warm up the clutch was slipping and i had to loosen the cable away off again, cant really understand why this was.
The clutch was still slipping quite bad but was getting better as time went on. I'm not sure its 100%.
I was thinking of changing the oil as i dont know what oil is in it.!!

Has anyone any other advice as i'm out racing again on Monday.!!
I guess my best bet would be to take the clutch casing off and have a look at the plates.

Sounds like the best plan. Measure all the plates to see if you have any frictions or steels getting to or below the service limit. Also, if you don't know the history of the current oil, new stuff won't hurt. Some oils have fancy stuff in them to help 'clean' things and/or allegedly further reduce friction within the engine than other oils. Sometimes these additives can cause wet clutches to slip.

Having said that, if you've been fiddling with the cable, you might find that the issue is somewhere between the lever and the clutch rod - ie the cable needs further adjustment or the release mechanism is a bit iffy and needs a good clean. There are a few other threads on here regarding clutch slip and how it may or may not be caused. Start with the cheapest option first in case it's something trivial!