Don't worry about having to pay for replacement panes - get the customer to sign a disclaimer first - you can't be responsible for the delapidated condition of the conservatory, if they want it cleaned they must take the risk.
Some years ago I was asked to quote to clean a job where the majority of the glass was over a pitched roof. The pitch was gentle enough not to make it dangerous to walk on, but the tiles were old clay ones, and many of them were cracked already. I told the owner that I wouldn't be responsible for any breakages as the only way to get at them (this was pre WFP) was to walk on the roof. He wouldn't accept that, saying that if I broke any, I should pay.
I didn't do the job
Cheers,
Ian