NHS organisations will have a policy for handling sickness absence both to ensure that employees aren't taking the mick and to safeguard their health. You should be able to get a copy of this policy and see what it says. Genuine illness must be treated fairly but if you are having regular spells of sickness absence which are impacting your job, the employer may want to consider changing your job or job hours so that you will be able to go back to full attendance.
Ask for a copy of the relevant policies.