I've removed MPlabX - trying to do some maintenance updates on on old project files using MPlab 8.92 - it was all working a couple of days ago and I haven't changed anything.
Update on the icon against all asm files - it shows them as being associated with Visual Studio and switching the association to MPASMWIN has no effect.
Sorted - I must have used Switcher when I was using MPlabX to set something and it messed up - I know that is only supposed to be to do with driver files, but switching everything back to MPlab 8 seems to have fixed it.