What is the name of this block, please?

[PotatoSoup]

Hiya, please can you help me?
These grey blocks are on the inner-side, with

red brick on the outside, of a house built in 1987. I'm trying to find out what they're called?
Many thanks in advance!

 

[Charlie George]

Celcon or Thermalite lightweight aerated block, every builders merchants will have stock.

 

[PotatoSoup]

You're a star!! Thank you so much