I've got 4 decent rear tyres in the shed that are in good order tread wise but seem to have gone hard and non-pliable over the course of time. Is there anything I can do to bring there 'soft compound' feel back?

You could try this stuff: Demon Tweeks Tyre Softener.

 

The real thing, or snake oil? I've no idea, but the MSA saw fit to ban it from use in control tyre classes.

2psi and 10 miles on the road flat out!!

Seems to work after ssdt.

Break a west Wales mans heart to chiuck them out !!!

Back in the 80's when the Michelin soft compound came out one of the other tyre makes used to soak the rear tires in mentholated spirit  

You could try this stuff: Demon Tweeks Tyre Softener.

 

The real thing, or snake oil? I've no idea, but the MSA saw fit to ban it from use in control tyre classes.

Kart guys etc swear by it

I have a friend who was race mechanic for an AMA Pro Dirt Track racer.  I'll ask him what they used for "Tire Dope", even though it was illegal!

There must be some sort of old wife's remedy out there!!

Mucoff mtb cleaner works,but not to good as new.

I have a rubber carb boot on a vintage bike that got so hard that it wouldn't fit back on. I soaked it in 3 parts rubbing alcohol,1 part wintergreen oil and it became soft and pliable.