Hi Ashley,
Are you being asked to clean the carpets or general clean as in dust, vac and polish?
If general cleaning, are you taking over from another cleaner, or has the office been left to become dusty/dirty, in which case you would need to consider a 'blitz' clean in order to make the place maintainable. Are you being requested to clean daily or weekly? Consider the time you need to dust, vac, clean sinks, toilets, wash down hard flooring. Quote at least 1/2 hour over and work on £12/hour.
You may wish to look on the General Cleaning section as you'll pick up more info than on Carpet Cleaning.
Hope this helps.
Best regards
Cheryl