There is lots of information on this on the dti site, but as with most things not exactly specific. My undersatanding is its an 11 hour rest break measured over 24hrs not between shifts.
I think I read somewhere its down to a health and safety prospective to weather an employee should attend work the next day, i.e will the person be to tied to safely carry out their job. Commen sence should prevail if someone is called out for a couple of hours during the night then maybe yes they should turn up for work the next day. But if they are called out for say 6 hours then it would be reasonable for them to not attend work as they will require rest.
The other thing to remember is an employer (unless otherwise stated in a contract ) does not have to pay an employee when they take this time off they may be entitled to some time off but unpaid as they have already been paid for the 6 hours.