Despite DDA all Employers really have to do is to comply with being 'fair' to their terms and conditions of their Emplyment contracts....i.e have they followed their sickness protocols with you.
It is illegal to give a person the sack for sickness if they have not followed legal guidelines such as doing return to work forms and giving counselling and warnings prior to the sack.
Sickness with an employee has to be managed, as unfortunately some people (not diabetics especially or anybody in particular) do take the mickey with absenteeism of any sort, and providing you Company has clear guidelines on their Employment Contract, despite the DDA you can be managed out of a job.
Diabetes is not an actual disability, but it is a manageable chronic illness. There are certain jobs like pilots and emergency services that will not take diabetics on medication and this is legal, but as far as I am aware from my dealings in longterm management of distrubtion centres and overseeing HR departments that technically speakking diabetics are not listed as coming under ticking the 'Are you disabled box' as unless you nead special hearing aids or special arrangements to be made regarding mobility then you are not technically disabled.
This is also highlighted in forms such as the disability living allowance-ie diabetes isn't reason enough to be given it.
If you are having excessive time off due to sickness or any type of absenteeism then providing your Company have a clear statement in their Employment Contract of how they deal with sickness, and providing they deal with you fairly and have made adaptations to workplaces if needed and lunchtimes for food and places to inject etc....then they are entitled to sack people with diabetes.