I have a garage at one end of a block of 1950's Marley (I think)
concrete garages - the things built from prefabricated pebble dashed
concrete sections. The corner post on the detached end of the garage
that holds the door lintle up, and has one side of the up-and-over
door frame (with springs) attached, is starting to fall apart from
frost damage and the reinforcing rods are exposed and rusting.

Any idea where I could get a replacement from? Are there companies
that could make one, given suitable drawings?

I don't think I could diy it in concrete, but maybe fabricate one out
of timber. Wouldn't look so good but may be the only alternative.