In reply to Kate Smith:
Can't help for the Cairngorms but we're over in Arrochar/Cowal this week and they are a total blooming nuisance lower down. Basically don't stop, keep looking for them and kill any that land pronto before they get a chance to bite. Once above 300-400m though they are pretty much absent and replaced by those annoying black flies, they don't bite thankfully!
Generally they tend to be worse in the north and west but it's been so hot I'd be amazed if the Cairngorms don't have any. If you camp high though you may be okay. Hopefully someone else can give you more local cleg report.
Enjoy your weekend wherever you decide to go.