Everything is fine in low gears but in 4th and 5th it feels like the clutch is slipping, rapid increase in revs with no increase in speed. It is worst at around 130km/h and is very difficult to get past this speed. Problem worsens with increasing temperature.

If it is not the clutch my only other thought is too much oil as the dipstick reads at the upper most line (checked correctly as per Haynes), is this possibly causing the problem? (Really dont want to have to drain fresh oil just to check plates if its not the problem, bike is for sale)

First things to do would be adjust the clutch cable at the engine side. If there's no adjustment left then replace the cable. I'd also say to drain the oil and replace the filter and use 10/40w semi-synthetic oil. Quick and easy and cheaper than changing clutch plates.

I have adjusted till my knuckles bled with no results, it is a new cable and fresh Castrol Oil bought from Honda.

Bump

Only reliable way to determine the problem is to disassemble the clutch and measure the plates and springs. You can reuse the oil.

Well I was hoping someone might be able to rule out or confirm that the oil level being too high is a problem but ultimatley I guess that's where this was always headed.

Oil level too high will probably not effect the clutch operation.

Siphon some out from the dipstick hole if you're unsure.