Neil you can catch them under ice but obviously to stack the odds in your favour try and pick a time when the conditions have been fairly stable for a few days and you will have em.
I was interested in the bites that Alan mentions. I first came across this on the Royalty fishery and I have found that it does tend to be quite a common occurrence on very hard fished stretches or swims.
Me and a mate had these whacking great bites that virtually pulled the rod off the rest without hooking the fish. We slashed away all day without actually catching one.
There is a way round this. Most anglers getting these type of bites are ledgering downstream. First way round this is to move your position and try upstreaming. Put on enough lead and get a big bend in your quiver and wait for it to spring back. I think Tony Miles has written loads of stuff on this...it really works.
Second way is to turn your end rig into a bolt rig set-up, hair rig the bait and semi fix the lead but if I had to choose I would go for the upstreaming.
