Another leaky disco.

Hi guys some advice needed, my td5 has started to leak just where the head linning meets the windscreen driver side), I have read this could be due to the drain hole at the top of the A pillar that meets the gutter, seems likely but the question is- is the drain blocked if so what is the best way to clear it, or is it better to seal the hole at the top as I have read in the search forum. Thanks in advance.
p.s no sunroofs fitted to this td5 and no aircon, ex commercial!