"In fact there are very few of the computers in an NHS trust that dont use software unavailable on linux , if you can unpack that string of negatives."
I think I've unpacked it correctly.
But let's make a slight change:
there are very few of the computers in an NHS trust that don't use functionality unavailable on Linux
As an example, if we discount Wine IE isn't available under Linux but other browsers are. It may well be that there are web-based applications that are tied to ancient versions of IE. That's something that has to be faced anyway, otherwise the NHS would be stuck on XP forever. When these are replaced there should be an insistence on cross-platform support so any browser can be used. The same insistence could be applied to any new web-based applications: if they're not sufficiently generic to run across multiple browsers the bid goes in the bin.