Just to update this post.
Thanks to everyone for your suggestions.
Eventually I went with John Keatley metals. (Thanks D A Godley).
They were happy to supply a few other small bits of steel I wanted (like 30mm lengths) and prices for the brass were very good.
They even saved me £20 Plus VAT on postage by discussing what I was using it for and suggesting an appropriate cut to the Brass.
Well recommended, and they even answer the phone!
John