Several questions. I assume you started this job in December 2007 - not 7th December 2008 (which was a Sunday).
If so you've been there just over one year (so have gained some protection from unfair dismissal).
If so, what time have you had off in total since December 2007 until now - including the 2 weeks for gallstones? Please indicate the number of occasions you have had off, and the total number of days.
If this is the only occasion of absence in 13 months and it was for an operation, you (probably) have a reasonable grievance. But I suspect there is more to this (in terms of previous absences).
Lakitu, I'm afraid, is incorrect. Certified absence does not protect you from disciplinery action for the total 'Package' of absence occasions. The bottom line is that if you are absent frequently you are not able to do your job. In the event of dismissal, the employer would probably use grounds of capability.