Clutch slip.

[BeardedShooterUK]

Before you go diving in there, check it is correctly adjusted, both at the push rod end and the level.  Then check the clutch is operating as it should, i.e. releasing.  They can stick/jam after periods of non use especially in could and damp conditions.  Get the bike up to operating temp then starting pumping the the clutch level, the operation should be smooth, light but positive if not, it's sticking.  Either the cable, and/or the release mechanism and or the plates/springs.

If the clutch is correctly adjusted and operating and it still slipping.   You're going to have to remove the side casing and check the operation of the plates.  Chances are they are sticking and not engaging properly giving you slip.  Before you start buying anythings, strip the clutch and check the palates for damage/wear/warping and then measure them.    If you find any are damaged/worn/warped or out of spec replace them.

Yes soaking the plates is a good idea, no the bolts and basket should be fine but you'll have to check, that includes the basket bearings, the springs you need to measure them, any out of spec replace them.  The cable should, unless it's been allowed to run dry be fine, just check for a smooth operation.

Thanks very much for this, might end up saving me some money, I just always assume worst case because that's my usual luck.
As far as cable adjustment goes, it's almost all the way out at the bars (the threaded adjuster bit) and the biting point is when the lever is pretty far out, so I assumed cable worn at the very least.
From researching, it's fine to remove the side casing with the bike on the side stand, without draining the oil?
I'll order a gasket for the casing to replace that after checking the basket and plates as well.
Do you happen to know of an online manual so I can check what plate/spring measurements should be?
I did have a Haynes manual, can't seem to find it after a house move.
Can't seem to find a correct model manual either, maybe I'm just not seeing it though.