I can't see how you'd ever get a block in. It only looks like a 20mm deep recess to me.
In the back like this more modern one:
That's my thinking too - and the thermal bridging seems much worse, not to mentioning hitting steel when you want to put up a curtain pole.
garyo posted a new thread.
What kind of lintel is this? If you look closely, it's not possible to set a block in at the back, and yet it appears to be a cavity...
450mm is a popular choice. Enough room for 2 courses of blocks inner, and 6 of bricks on the outer skin.
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