Having diabetes doesn't mean you can't be dismissed for taking time off sick due to it, but your employer would need to give you special leeway compared to someone who doesn't have an illness classed as a disability under the equality act. So for example, if the normal company rules would be to dismiss someone after x% absence, they may look to increase this in the case of someone with diabetes related absence. It is called a reasonable adjustment. Obviously there would come a point where it wasn't reasonable for the employer to make any more allowances, for example if you were off a great deal -at that point a dismissal would probably be fair. In this case, if you've had one long absence I'd say that they should allow a reasonable amount of short term absence, but if you were off for another several months, say in the next year or so then they may have a case for dismissing you as they could not be expected to put up with such a long absence again.