Just speaking from personal experience, I have had Virgin Queens mate very well in mid-March before (and late September, for that matter). There always seem to be residual pools of drones. Furthermore it has occurred to me more than once whether the proportion of drone laying colonies exiting winter (c.a. 5% at a guess) helps to get early Queens mated. Whilst the weather is admittedly patchy (here in the Midlands), there are now plenty of mild/pleasant spells (today/now being an example), and I don't think that is now a big factor. Almost all my colonies have healthy, capped drone brood on the outer edge of the brood nest, and, whilst it will still be a couple of weeks until those emerge and are sexually mature, I'd personally be more than confident of a VQ flying today getting mated. Just my thoughts, anyway. The bees always find a way.