I presume your rack is the one by
H&H Design. It is indeed structurally sound unlike most of the aftermarket racks offered for Brompton. My moving of the sideways roller wheel onto that rack would be quite a challenge though. I think will look into just replacing the stays of the original Brompton rack with steel ones.
I bought mine from a private guy named Maxim Nreg (or something simmilar) on eBay, but I cannot find his listing there any more. H&H make aesthetically pleasing products, but I had some problems with their structural strength. Like I said, replacing the rack was a straightforward ten minute job, no complications involved.