Clutch slip.

[darrsi]

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.

If you've not done it before then definitely do a proper adjustment of the cable to begin with, as it may be your problem.
It is worth buying a 12mm deep set ring spanner to adjust the lower end properly, it's not 100% necessary but certainly makes things easier to do. Just pull off the rubber cap, and behind it is a 12mm nut which also has a screw in the middle of it.
Before touching the lower end make sure you undo the locking nut on the lever then screw the adjuster inwards as far as you can, making the lever become floppy.
Undo the large nut (preferably with a 6 sided socket if you have one) then when the screw in the middle becomes loose enough to rotate you need to gently turn it clockwise until it stops, then turn it anti clockwise one quarter of a turn and while holding the screw in position nip up the outer big screw with the ring spanner until it feels firm.
There's no need to massively overtighten that outer nut, because it will just make it difficult to undo next time.
I just nip it up firmly, or you can use a torque wrench if you have one.
Then go back to the top and readjust leaving a tiny bit of play in the lever before using the locking wheel to finish up.

If you do ever need to change the clutch cable, make sure you use strong tape or a cable tie to join the new one to the bottom of the old one, then pull the cables upwards so that the new one uses exactly the same path as the old one. Don't try and route the new one yourself because not only is that a more difficult way of doing things but chances are you'll get it wrong anyway, and that is simply not worth the risk.