In reply to gobbler:
Can I ask what sort of experience do you have with winter climbing / ice climbing already? Mid to late March is getting a bit late for mixed climbing and is typically ice climbing time of year high on the Ben.
That said the ridge routes on the Ben will probably be doable in mostly neve conditions but with some mixed climbing,
If you specifically want buttress climbing on snowed up rock (with some neve / ice / frozen turf) then normaly mid jan to late feb is the most reliable time and normally the Cairngorms is the best bet. The most reliable time for getting anything done in winter conditions is probably Mid feb to April on Ben Nevis, but partially as there is normally a build up of snow and ice which is resistant to thaw, which is why neve and ice climbing is often later in the season.
Post edited at 23:20