TCX cable arrived today (for £8). However, I'm starting to suspect the clutch plates more and more now, as on hard ride yesterday it kept slipping lots when changing up and accelerating hard. Did stop many times to tweak cable adjustment, just to make sure it wasn't the cable/lever preventing the clutch fully engage - but no difference.
Just pulled off sprocket cover and yep, also faced with tonnes of oily greasy grit and dirt too. Cleaned the lot out, possibly this was hindering the clutch cable mechanism too. Original oem cable I fitted 2 years ago, still appears in reasonable nick actually, no fraying, smooth action etc. Either way, blagged some new EBC clutch plates & springs from Bryn on here, so I'll fit those one of these coming weekends and see how it goes. I guess at 50k the original clutch plates have probably seen better times.