We've just had an old gas fire removed from chimney breast. Behind is bricked up infront of fireplace with whith whjat looks like breeze blocks but could it be asbestos? its a grey white colour.

do you know how long ago this was done as thermal blocks are grey white but only around 15 years old

Leave it be and do not disturb it and cover the hole up. Then tomorrow or when ever get in touch with your environmental or a reputable builder because if it is asbestos it must not be disturbed because of the spores. It has to removed in a environmently safe fashion by experts and will then be bagged and disposed of in a licenced tip.

Stick your clothes into a bag and dispose of them and go and have a shower.

It could be cement board.
There is still loads of asbestos in old houses but unless you turn it to dust and breath it in...........

Not asbestos.

asbestos is sheets not blocks

yes and if i were you i would keep away from that stuff my husband died with abestos exposer.