I thought I might cover the bucket with clingfilm as I'm really curious about what they are. If they hatch into something horrific they can, and will, be well sprayed with insecticide. I don't think I'll put them down the drain - they might like it down there.
Whatever laid them in the first place is obviously around, or has been. We've had the usual flies, bluebottles and midges, a few bees and the odd wasp but I haven't noticed anything unusual.
I will keep you posted.