Andrew Mawson wrote

What I'm trying to find out is what is used as the spacer in lateral
expansion joints in long walls whilst they are being built. The two sections
of wall butt close together end to end, are prevented from moving sidewise
by normal wall ties fully bedded in a mortar joint on one wall section, but
wrapped in dpc in the other wall section to allow it to slide along the
length of the wall. Finally the joint is mastic filled to make it
weatherproof, but during building 'something' is used to ensure an 8mm gap
in the mortar.

Logically it has to be firm enough to keep the mortar out while setting the
bricks, but able to be squashed as the wall length changes with expansion.

If I cannot find what is 'proper' I intend to use slices of expanded
polystyrene.

Andrew - I've had several occasions lately when nobody has replied to messages.
I gave the answer to this question at 11.10pm last night and I can see it on my
machine, but I'm wondering if I'm the only one who can! ) Could somebody
confirm they can see it too please.

BTW - the answer is Flexcell - it's a bitumen-impregnated soft board
http://www.sundeala.co.uk/flexcell.htm

Peter