I've just received December's Gardening Which? magazine, featuring a test of these lights and every one of thirteen tested was judged unsuitable for winter use if you rely on the sun to charge them. The only way they say to ensure they stay on for more than a few hours is to get a set with replaceable batteries and fully charge these with a battery charger. They recommend you hang on to your receipt if you have bought some, if they dont perform as described you can ask for a refund.
As a hint of how bad they are, the best you can hope for is 6 hours lit if the solar panel is charged in full sun, the worst a few minutes if overcast. When on they are quite dim too.
Sorry to upset those who've already got solar lights, but I felt I should share the information so that you have plenty of time to do something about them before Christmas.