Can a fused spur be fitted into ring main

[Taylortwocities]

And in case where the fuse should ever pop, tHe main ring will continue to work uneffected?

I already told you that - the ring will be unaffected

Generally what is better, spurring a FCU from a socket on the ring or adding the FCU to the ring

Whatever is easier to wire

Secondly and slightly unrelated, can round junction boxes be hidden behind behind a plaster skim?

NO, not EVER. All screwed connections must remain accessible. There really should be no reason why any junction needs to be hidden. If it really must then you'll need a "maintenance free" junction box like this

 

[EFLImpudence]

Just to confirm and clarify -

It is the branch cable which is the 'spur' whether fused or not.

The accessory containing the fuse is a 'Fused Connector Unit'.
This is sometimes called a spur by those who should know better but it is not.

What are you planning to connect at the end of the spur?
You may not need an FCU in a prominent position next to the socket. It could be positioned elsewhere.