Posted: 14/07/2014 at 13:50
They look like black aphids - the sticky liquid will be the honeydew which they excrete - you'll probably find ants going up and down the tree to the aphids - they actually 'farm' the aphids and 'milk' them for the sweet honeydew. A black sooty mould will then grow on the honeydew, discolouring it.
The best remedy is to encourage wildlife into your garden, ladybirds and bluetits especially - the ones in our garden simply hoover up the aphids, especially at this time of year when there are lots of fledglings about.
Hang some bird feeders in or near your tree to encourage the bluetits and they'll soon find the aphids.