Written by a solicitor;
"An employee who is off sick for a prolonged period of time can be fairly dismissed if they are unlikely to be well enough to return to work within a reasonable time.
What amounts to a reasonable time depends on the nature of the job, the specific difficulties encountered by the employer in covering for the absence and the size and administrative resources of the employer"
and
"An employee who has frequent short term absences can be dismissed fairly even if no one absence would be long enough to justify dismissal on its own. Before an employer can dismiss for this reason the employee needs to be told what level of sickness absence is not acceptable and given the opportunity to improve their sickness absence levels"
Source:
http://www.rightsatwork.co.uk/employment-law/sickness-absences.html
Links to relevant legislation can be found here:
http://www.hse.gov.uk/sicknessabsence/legalguidance.htm