It may be very obvious you have a problem with sickness absence with a particular employee. However, in order to show consistency you should, as a matter of good employment law practice, review all of your employees attendance records at regular intervals. You need to chose a period - for example, six months and then ascertain the patterns of attendance. The most worrying aspect for an employer is lots of separate days off, especially where many of these days are Mondays or Fridays, over Wimbledon or other suspicious dates. Once you become aware of this type of pattern, you will need to talk to the employee concerned to try to improve the situation. It would be normal to start this informally and take a supportive approach - you cannot start with a disciplinary approach until you have carried out an investigation if you want to avoid an employment law claim.
It may be the case that the employee will genuinely have problem - perhaps one that is not sickness but for which you can feel sympathy - such as childcare problems. It may be the case that the employee has a genuine illness, such as IBS, which accounts for the pattern. If this is the case, you should try to work with them to find a solution. You should also consider if there has been bullying or harassment at work or if they are simply "stressed". Again, it may be possible to work within these constraints and notice a considerable improvement in attendance rates. Some employers offer a bonus for reduced sick days taken. Some employers focus their efforts on helping employees achieve a work/life balance - perhaps arranging for staff to go home early once a month or come in late, with slightly more flexible hours. You could see if you could arrange a membership deal with a local gym, to promote good health and reduce stress in your workers. You could, with a persistent case of absenteeism, offer private treatment instead of a bonus.
If none of this works, then you need to ask the employee for permission to see their medical records and you may consider asking them to go to your own health specialist. If the problem is serious and despite your best efforts there has been no improvement then, provided you have dealt with the requirements of the Disability Discrimination Act, you should move to dismiss them for unacceptable attendance.