Thanks for the kind offer, I've actually already made up a repair for it now. The indent down the centre was a bit awkward to do and doesn't quite follow the same line as before, but I'm fairly happy with the outcome (although not happy with my painting skills!)
In the end the whole 2mm plate underneath the thicker brace part was rotted right the way back so had to be cut out, and the thicker plate above was also 90% rust. That part is 4mm plate but welded ok with the edges chamfered.

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