I would look at the attendance by year then
1 - Insist that parents ring in each day of absence and initiate first day calling and insist that parents write in to cover absence
2 - Print off the registers for last half term and look at patterns, are they having every Monday off, is it PE day, do they always have days off after a holiday, does the attendance % look poor for a student but then emerges it was due to chicken pox earlier in the year and student has had 1 day off since.
3 -Weekly ask admin to telephone home for missing absence notes (see above) great way in if done in a pleasant manner as parents more likely to divulge any issues. Also parents begin to realise that school is monitoring attendance.
4 - split the attendance into attendance % categories and get to know which students are in each. The idea of this is that you can 'target' groups of students, monitor the severity of letter home eg under 85% EWO action but above 85% a montioring period for improvement. After moitoring period write home again to say improved or not.
5 - Don't focus on the the hardcore 3 or 4 familes that have persistent poor attendance- hand them over to the EWO, as a school aim to try and prevent the student having a day off because they have a headache, or who have woken up late or have a dentist appointment and parents think there is no point sending them in. Better to be late than not in at all - then work on the lates.
6 - After the EWO families then choose the next say 5 and work with these for intense work eg home visits
Sorry this is all 'office' stuff but I feel that Primary schools face a bigger battle with attendance then secondaries as most students cannot get to school without the parent. At least in secondaries (with my experience) if the student wanted to come to school they could get themselves there. Little ones unfortunatley cannot.
Good luck :)