Hi V
I would add these sites to E's suggestions.
Crummock http://www.themodernantiquarian.com/site/7823
Grasmere http://www.themodernantiquarian.com/site/8711
Tortie http://www.themodernantiquarian.com/site/2443

I would also suggest that this book, despite the inclusion of a couple of iffy sites including 'that spiral,' could be a useful addition to your Cumbrian library - http://www.amazon.co.uk/Prehistoric-Rock-Cumbria-Stan-Beckensall/dp/0752425269/ref=sr_11_1/202-6284786-8479068?ie=UTF8

If your looking to seeking out a new site then I would suggest having a scramble around the glacially smoothed outcrops at the head and foot of a lake, preferably in places where possible prehistoric trackways may have existed. There are definitely carvings out there just waiting to be discovered.

Happy Hunting
fitz