You don't know if there will be anything, is the truthful answer. One day in June-July time I went over and had one hawk at 3pm (I'd been sat on the hill since 9.30am) and that was it all day. Maybe I was unlucky then or lucky for this trip, who knows.
They normally fly during daylight between Monday to Friday, excluding public holidays etc. From what I can gather you get more traffic in the Mach Loop in Wales, F-15's from Lakenheath etc. This isn't to say the American's don't visit the lakes at all, just seems more common they will fly to Wales.
It's something you have to do! :pilotfly:
Edit: Found 2 more 'acceptable' (terribly backlit) photos of the German Tornado's and a photo of the view I had on the 2nd day!
The view from Great Howe, looking over Thirlmere Reservoir and towards Dunmail Rise (were I was the first day).
A few more are on my gallery.