Is there any way you could fit a wall bolted, 'triangular' shaped,
steel brace in the middle or do you need an uncluttered space?

I'd need to drill a very big hole through the washing machine to accommodate
the brace. It might prevent the drum rotating, too.

Seriously, though. I've been told that a 40mm wooden beam across the front
is sufficient. I just want to know if I can reduce this using a metal beam
(or several) as I don't have 40mm to play with. There is absolutely no way
that any part of the structure can extend more than 25mm below the bottom of
the worktop over the 121cm span.

Christian.