Annual tax return HELP

Hi
Can anyone advise the triggers that a PAYE employee would be requested to complete an annual tax return?
I know that if one makes an AVC to a pension scheme outside the payroll system & claims the tax back, this could be a trigger.
My question is if one was to claim the tax on medical expenses back, could this also trigger it?

thanks

If you are only a PAYE worker, then you would just need to complete the MED 1 where you would put in your P60 details.

Do you hold any investments? I had opened a Rabo Investment account last year and had to file a tax return this year. Also if you might have substantial deposits this may also trigger it as you would need to disclose your interest earned, though there would be no additional liability