It appears you're moving in the right direction Alex. When discussing pricing over the phone, you are only able to give an estimate. If you visit the site, you can give a written quotation. As mentioned by others here, ask questions. Type of pile? Type of construction and backing material? Cleaned before? Colour? Stains? Size of room? Hopefully, Sir/Madam won't be able to answer all of your questions. You've shown that you are a caring professional, so you can now come back with something like: From the information you've been able to give, I would eatimate a cost in the region of £65. I can visit your home to carry out any tests that would be required in order to clean your carpets safely and confirm the price as a written quotation, guarenteed for 30 days.
Safe and happy cleaning:)
Ken