Thanks for all the replies.

The problem relates to the ongoing saga of my mother's woes with
Anglian Windows which I've posted about here before.

They removed a window, didn't prop it, didn't install a lintel, and a
triangle of bricks came away. The BCO came out and said a strong wind
or hard frost could bring the lot down.

Their "best brickie" came out and rebuilt it after installing the
angle iron - and you've never seen such a f***ing mess. Like some
rustic brickwork in Santa's grotto - not a straight line anywhere. A
neighbour described the wall as having a belly on it.

He came out and did it again. By this time the bricks were getting
pretty damaged from being taken apart and cleaned up twice. He'd only
brought a few new bricks (which weren't an exact match), didn't want
to go and buy any more and was considering going round the
neighbourhood in the hopes that someone might have some in their
garden that he could nick. In the end he botched it with the damaged
bricks. Then Anglian (after much ear-ache from me) paid for a company
called Construction Cosmetics to come out and paint the new bricks to
look weathered and blend in with the old. CC said that the bricks
were sufficiently damaged that it was hard to tell where brick ends
and mortar begins, but give it a few weeks to weather and see what it
looks like.

Actually their paintwork looks pretty good but it hasn't concealed the
fact the brickwork is still sh1t.

And the lintel looks like it's deflecting.

I've juts phoned my local BCO and he said that 12 feet is "hellishly
wide" and that he can say without seeing it that it won't be
adequate. He also said that it should have been done with a prior
building notice and that re-doing it should be done after submitting a
building notice.

As far as what should go in there, Dave Plowman was bang on: BCO said
get in a structural engineer to do the proper calcs.

Tim, thanks for that link to Catnic. On page 41 they do have the sort
of U-shaped lintels I was thinking of -- although on the same page
they also have something that looks like angle iron, presumably a
beefed up version though. I might give their tech help a call
tomorrow.