Well, it is clearly a non compliance against current regs. It may well have been allowed when it went in,though - for quite a while concentric cables were permitted.

Is it a dangerous non-compliance ?  probably not, especially if the cable route is either obvious or mostly in the 'landlords area' where random tenants should not be hanging pictures anyway.  And in practice assuming neutral is the outer, then any nail accidents will result in power going off smartly. We'd not worry if it was a network operator's cable after all.

If it is too accessible at the ends where it is single insulated, can things be sleeved or 'wizards hatted' or protected with mini-trunking or similar ?

Mike.

I agree.  One point to watch though if you are replacing whatever is on the end is that the terminal that receives the solid Al core needs to have been designed with that in mind.  I dont have the standards to hand but I would definately check before terminating it in anything different to the original.  DNO terminals for this sort of thing are mostly relatively heavy copper blocks with multiple screws which are tightened to required torque.  I doubt that you would get a decent connection with any longevity with a cage type terminal.

I agree.  One point to watch though if you are replacing whatever is on the end is that the terminal that receives the solid Al core needs to have been designed with that in mind.  I dont have the standards to hand but I would definately check before terminating it in anything different to the original.  DNO terminals for this sort of thing are mostly relatively heavy copper blocks with multiple screws which are tightened to required torque.  I doubt that you would get a decent connection with any longevity with a cage type terminal.