Although there is no max duty period limit, there is the weekly, fortnightly & monthly limits. Doing a duty such as Viking9 has shown would be in the order of 15-16 hours, or over a quarter of your weekly duty hour max in one hit.
Secondly, the company would have a 'duty of care' over their employees under SHEQ after completing such a long duty. Normally hotac would be provided either in MAN before positioning back or in LGW if positioning straight back.