Yeah, 24 hours notice is always the figure I hear.
Yep, 24 hours notice is the legal requirement. If they insist, inform them that the 'unreasonable interruption'' is harassment, and you are being 'denied quiet enjoyment of the property'.
The agents are usually allowed access to the property at a time suitable to yourself, within the time period stated in the tenancy (usually the last month or two), anything outside of those time frames they can whistle for!