Hi Andy,
Do you mean you are being made redundant, or that you have been dismissed? You can only be made redundant when your job no longer exists, in which case it doesn't matter what medical problems you may have because there is simply no longer a job for you to do. If they have selected you for redundancy (while keeping on someone else in a similar job) and have told you it is because of your health, or if they are sacking you because of your diabetes then they are breaking the law and your first action should be to seek professional advice (your union or a solicitor specialising in employment law).
Alternatively see your local branch of the CAB (Citizen's Advice Bureau). They deal with cases like this every day and will advise you on what your rights are, what you can do about it, and will even fight your case for you if you want them to. They also have access to specialist legal advice (and free to you).