What is the lining of a woodburner made of
the back plate in mine has cracked in half and need to get a new one

Varies. Mine are cast plates, others are fire brick material which you can buy in sheet form and cut to required size.
Mine was an absolute **** of a job to replace that lost me the skin on several knuckles.

It seems like a very lightweight material of little strength
but glows redhot when in use like some sort of fibreboard

Firebrick sheet.
Probably cheaper at a local supplier.

Vermiculite board. It is light, brittle and hard to find at a good price

Troutie meritt and fryers sell vermiculite sheets which can be cut to size.

You can also get a bit more life out of them by bonding them back together with some fire cement.

Yup firebricks going to need to replace mine in the spring.

Just because a firebrick has cracked doesn’t mean you need to replace it – it will still work fine.

Only needs replacing when chunks start falling off.

Cheers that Vermiculite looks like the very stuff
Wharfedale never thought about Merrits
the back piece broke in half and I bolted it together with steel plates
but the steel on the flame side has now disappeared

Merritt’s is the best! Open til 12 today.