Maybe this page will help you - I see the coastlines are from a datasource called PGS and there is a page for documenting errors you observe:
osm.wiki/index.php/PGS_errata I suspect you'll have to do the fixing :)

Best wishes!

Thanks guys. Thanks also for the comparison check - I find some strange behaviour about which scales and tiles have the error. I'm going to probably finish up that area today just to make mine more complete around that problem :)

Cool - looking forward to trying it on my Symbian phone, with my own data. Thx!

Arg.. here's a better link:
http://tinyurl.com/2jlh4w