Surprised it's a DNM unit, I thought all the airshocks they've fitted for some years were DT Swiss units ? (used to be SSD225) - the ones they fit now seem to be DTSwiss XM180, and rather expensive* in the UK
*That said:
http://www.bike24.com/p221984.html140euros
- I think the length you want is 165mm, can't remember if 37.5mm is the correct travel
There's a few other dtswiss options on the same page, so you might find them in the UK at a better price than the XM180 is listed at.
BTW - assuming you order the mounts/need the seat end machining to size 'cos of the mount type, I'd suggest measure the distance between the mounting plates with some vernier calipers. Mine splay from bottom to top, so bolting up the mount pulls them together. I queried HPVelo on this and they said it was within tolerance, but TBH I'd rather fit something that was nearer the real dimension. They're not all like that, certainly one poster on BROL found his to be the correct spacing and parallel. I've got an SD225 to fit, but I'll probably spend a few extra quid to get the correct size made up (which is a shame, cos when I bought it from CRC some years ago the mounts were included in the price).