> 1) I'd like to get in at least one route at Red Rocks. Am I likely to get anything done at The Gallery at any time of day in August?
Probably too hot. Maybe look at Black Corridor for some shade.
http://www.mountainproject.com/v/black-corridor/105732030
> 2) I'm planning to travel via Bishop to Mammoth (should be cooler than ORG?). If I'm lucky I might snatch an hour or 2 in Bishop. Is there any roadside bouldering at Bishop which is worth trying without a mat / spotter?
Happies or Sads for sure. Or try out Deadmans Summit or the Bachar Boulders. Both are basically roadside.
http://www.mountainproject.com/v/deadman-summit/106159406
http://www.mountainproject.com/v/bachar-boulders/106047133
> 3) I've bought the Mammoth guide and Clark Canyon looks good. How popular / polished is it and will I need anything more than the usual family saloon hire car to get to the crags.
Clark is good and not really polished. All sport and pretty short. Any car will be able to get in there.
> 4) After Mammoth I'm planning to go to Yosemite for a day or so and wondered if there was any single pitch sport or easy low ball bouldering worth a look, or should I just stick to touristy stuff?
Yosemite Valley will be hot, but go and at least look at the Camp 4 bouldering. Approach is maybe 1 minute from the car. Not sure on the sport stuff, but there are bolted climbs right near the road.
> 5)If there's nothing suitable in Yosemite, should I try Tuolomne instead?
Do both. Tuolumne is on the way to the Valley from Bishop/Mammoth. Roadside bouldering all over, and some good topropes and bolted climbs all near the road.