Clutch slip.

[BeardedShooterUK]

From your description i would start with properly adjusting the clutch cable, by screwing in the top end first so that the lever becomes loose, adjust the lower end, then re-adjust the top end again until it's correct, as you never mentioned any adjustment lower down.
Expect the possibility you might just need a new cable though if there's no improvement, but i suspect there might be if it's done correctly.

As the bike's been inactive for a long time you might want to consider an oil change as well, unless you know the proper history of it?
My bike had just been off the road for 18 months due to a personal injury, but my oil had been changed 4 weeks before then so i knew it was all good.

This may sound daft although it's really not, make sure you buy proper motorcycle oil, do not use car oil as some have additives in them that can make the clutch slip on a bike.

Doesn't sound daft at all, I appreciate and value any and all input.
I haven't made any adjustments yet to the bottom end of the cable, but I do have a fresh one to go on anyway.
However, I did remove the clutch cover and plates to check the thickness remaining and they are at 2.76mm, which is a hair under the manuals 'limit'
I wouldn't have thought it was cause slip so close to the limit, but it is indeed out of spec.
The steel plates aren't blue or burned in anyway though and still within spec.
Oil and filter was done not too long ago by myself using Castrol Power1 4T motorcycle oil... Appreciate that as a suggestion though as I'm sure plenty have just grabbed a bottle of oil off Halfords shelf and put it in... I also did a coolant change as well.
Going to buy some EBC friction plates on payday.
I don't know if this is at all relevant to a clutch slipping but 6th gear has a bit of a whine as well, then again it has done for a long time and was told it was normal.