In reply to tehmarks:
I thought exactly the same thing reading the original post. Does 'double' mean half or twin?
To the OP, I'd say it depends on the route. Straight up gully type route, the single will probably be fine. Just take loads of slings to keep the rope running as straight as possible and be aware that your gear is going to be more prone to side pull. Eg if plugging both sides of a gully alternately with gear - more critical in winter since cracks can become icy.
Routes that wonder around, better to use the half ropes, despite no dry treatment. Less drag. Just be prepared for them icing up and becoming hard to handle.
Wouldn't say half ropes are critical for abseil descent in the Norries as the routes aren't long enough to add more than probably a couple of abs. Buttresses over 300m, half ropes are potentially going to save you a lot of time should you need to bail. Other option is to carry a second skinny rope in addition to the single rope just for abseiling. The likes of Will Gadd advocate this approach but it really depends on the route characteristics (he's mainly climbing ice and bolted mixed, where it's much easier to keep the rope running straight).
If you're finding that you're doing enough routes that justify the half ropes but the set you have are becoming a nightmare to handle then you have a good reason to buy some dry half ropes. I'd stick with what you've got though until you've demonstrated a need for another set of ropes. You might prefer the single. Easier to handle one rope for example. Personally I prefer halves on all but straight up ice.