Thanks all above. I had no idea there were so many aftermarket mounting options, although some of them do look a bit vulnerable for MTB, (but a pretty good option for roadies). I assume you could reverse mount one of the above as suggested, so it would sit above the stem.
I would prefer, if possible, to keep it on top of the stem itself, although if pushed I could stick it on the bars (adding to the clutter, but giving me many more options for cheaper stems – recently got a Hussafelt 40mm for the wife for circa £24).
Although the Hopes etc look smart, I find the whole stem testing process pretty random – my Thompson was a shorter stem than the stock one that came with the bike, but I had no idea how short to go. I guess they just have to go in the parts bin and ebay! Is there a huge difference between 100mm and 70mm? I am a bit worried less than 60mm will be a bit drastic.