Not sure if the seller is willing to ship to the UK: https://www.ebay.de/itm/Edelstahl-Adapter-fur-den-Brompton-Tragerblock-carrier-block/402480013181So, it turns out the mount has failed from the top right corner - I've contacted the seller but I imagine its an issue inherent to the design as it's a 3D printed mount.
Are there any better mounts for sale?
However - the shopping basket made by Brompton is a valid alternative in my eyes. Proven to work since 30 years, less rattle, adjustable in size, lasts forever. Immensely practical (and can even be overfilled using a rain cover from another Brompton bag). Possibly even cheaper if you value your time and and count the various materials and cost involved to come to a home made solution... https://brilliantbikes.co.uk/brompton-bags/3492-brompton-borough-basket-bag.html You might even be able to grab an older model of the basket from a dealer for a cheaper price as this version got only invented this year and the older version may still float around.