In reply to CJD:
last year i climbed a couple of times in a very short pair because it was hot and that was all i had, but they weren't much longer than the harness and i had to keep pulling them down under the leg straps.
This year I found an old pair of combats/jeans and chopped the legs off above the knee. They're really comfy and hardwearing and because they're about 8 years old they're high waisted enough to sit comfortably under the harness.
Only problem is that when i wear shorts i invariably manage to go home with various scrapes and bruises and the people at work then tell me i look like i've been fighting!