52 weeks to
29 March
2013
52 weeks to
28 March
2014
52 weeks to
27 March
2015
52 weeks to
1 April
2016
52 weeks to
31 March
2017
Revenue growth+1.0%+7.9%+6.9%+1.7%+7.2%
Gross margin54.8%53.7%53.2%53.2%51.0%
Operating margin**8.8%8.3%8.4%8.3%7.0%
Underlying Group EBITDA***£103.4m£101.1m£109.9m£114.6m£108.7m
Net debt***(£110.6m)(£99.6m)(£61.8m)(£47.9m)(£85.9m)

* The statutory 53-week period to 3 April 2015 comprises results that are non-comparable to the 52-week periods reported in other years. To provide a more meaningful comparison, the above tables include the pro forma 52 weeks to 27 March 2015.

** Operating margin is defined as results from operating activities before non-recurring items as a percentage of revenue.

*** These alternative performance measures are defined on the key performance indicators page.