Birch trees dying - what to do?

I planted a few silver birch trees in our garden about six years ago and they've been growing well and looking beautiful until this year. I've noticed to my dismay that only a few leaves have appeared on the top-most branches of about half of them, and the odd leaf here and there throughout the mid/lower branches. Some of the leafless branches are brittle, and dead. All the other trees (ash, beech, oak and various ornamental) seem to be fine.

Can anyone recommend a good Irish/UK site that might help me identify the problem?

Or would it be worthwhile to pay a tree surgeon, or pro-gardener to have a look? Would that be expensive?

Try the forum on gardenplansireland.com.
Leo