In reply to Oo: If you happy leading long multi-pitch mountain rock routes and can also competently navigate in low visibility then you have the essential skills needed to head out Winter Mountaineering, assuming you take a conservative approach to everything. [Conversely, if you wouldn't feel 100% happy heading up a rock route on big Welsh mountain crag as the more experienced of a pair, then probably book a course or find someone to mentor you.]
If you go out on days with better weather forecasts, depart early and allow plenty of time, a stick to shorter, easier routes that are accessible and known to be in condition and are paranoid about avalanche conditions and reading the SAIS reports (if in any doubt completely avoid terrain rated category 3+, you can always practice skills just above the snowline and head to the pub early) then you are very unlikely to have a major epic. As long as you initially have that mindset, then experienced summer climbers/hillwalkers should have no problem giving it a go.
However, it is worth adding that there is a world of difference between being out in Scotland in late December and early March. The later is often relaxing and thoroughly enjoyable, the former rarely feels like anything other than stressful. As a novice, the later in the season you go out the more leeway you will have as regards the daylight and the more likely you are to be able to fully develop your skills as part of a full and rewarding day out.
Easy Winter climbing is not far removed from doing Grade 3 scrambles and climbing easy multi-pitch routes in big boots. You do nothing particularly new and you just need to learn how not to trip over your crampons plus how to belay on steep snow rather than just using rock anchors.
The basics you need to master before doing anything other than easy walking are not complicated. But, if you don't get instruction then you will need to put aside dedicated time to experiment out on the hill before ending up stuck below a cornice with a 100m+ runout below you - which can often occur even on a Grade I gully. I would say the basics are:
- moving up, down and sideways on steep snow slopes in crampons.
- moving over ice/snow covered bouldery/rocky terrain in crampons.
- mastering a basic ice-axe arrest (See Youtube videos or the BMC Winter Skills DVD. If in doubt, download a clip and take out with you on the hill on a smart phone.)
- digging a horizontal axe belay combined with a bucket seat and body belaying. (Again, watching a decent video and practicing 2-3 times is easy BUT time consuming.)
- direct belays around a boulder/spike plus Italian Hitch belays (may be revision, if not then again read up on them).
optionally:
- placing a Deadman (if bought/borrowed)
These basic Winter skills, combined with the ability to place slings, nuts/hexes and standard rock climbing belaying will keep you going for dozens of brilliant days out on grade I-II terrain. The progression to grade III and harder will then be fairly natural, if not without a few slightly scary moments.