Clutch slipping after 6000 miles

When I had the 2.0 16v put it in, I got a new clutch as well -
RTS Clutch Kit £291according to the receipt.
It's already started to slip (and stink) despite only having done 6000 miles.
Is the standard Sachs one okay for the engine power?
 

Did you replace the cable?  You may just need to adjust the cable.

I haven't tried that.
It has the 10mm space at the gearbox side.
I'm not sure how old it is… I don't see it listed when the engine was changed.

I guess it could be slow to slide back to position.

I took it for a run to test. It seems intermittent.

I left it in 3rd and floored it a few times, and it didn't slip once. 3rd is where it has slipped in the past. I guess it's worth taking the clutch cable out to have a look.

You could remove the cable and see if it is really really free to travel.  

I have seen cables that break internally that unwind and the broken strands can catch on the inner cable housing and bind.  (Well Parking brake cables) causing the brakes or clutch to stay on.  

I have had a clutch cable in my 90, start cutting the feed through so I replaced it.

I've taken it out and , although it could do with replacing, it's not sticking anywhere. I flushed it through with WD then some oil so it's a bit smoother now. I'll pop it back tomorrow as I can't find a replacement right now.