The legal number of hours between shifts is determined by the Working Time Regulations 1998.
They state that the minimum rest period in a 24-hour period should not be less than 11 consecutive hours. In general, workers are entitled to at least 11 hours rest per day, at least one day off each week and a rest break during the shift if it is longer than six hours.
Opt Out agreement
An employer cannot insist that a worker works more than 48 hours per week on average. Any more than this are voluntary and subject to an Opt Out agreement.
Note that for night workers, there is a maximum of 8 hours work in any 24 hours on average and a right to free health assessments.
Mobile workers are subject to a variation of these rules under the Road Transport (Working Time) Regulations.
You can visit the Department for Business Innovation & Skills for more information on the Working Time Regulations and the rules for averaging out the above time limits over a period of time.