The SR3101 plastic version is on at Bykebitz for £47.24 including postage if you use the forum discount. The SRA3101 metal version is going to be £91.08. The plastic one has the infill which hides the honeycomb nylon, and looks very neat.
Colin, your M5 plate looks like the top of a Wingrack system, and would not really fit on the adapter, and you can't seem to buy that adapter separately in any case.
If I hadn't already got the metal one, and if the top box was going to be a near-permanent fixture, it'd be the £47.24 plastic job, leaving £43 free for... well, anything costing about £43, really.
Here's the metal version to compare visually against DLste's picture of the plastic one.