Umm...when the connection drops, how did you connect again? Do you have to restart your sky hub or powerline adapters? Do you have a Windows PC? You can try to disable power-saving....

Can you use your dlink utility and rename both DHP-306AV and DHP-346AV to be the same first. Test if your dlink powerline adapters work or not. Connect one to your router and another one to your PC....

You got two ER604W? As you said, when vpn connection drops, it doesn't reconnect automatically, but how did you re-establish the connection?

Did you get it solved? If not, submit a ticket here directly. I have no much time to read such a long post...lol :D http://ticket.tp-link.com/index.php?/Tickets/Submit

You should describe what you've done in details first. Otherwise, they may keep asking you to repeat what you've done. Anyway, if you get it solved finally, pls share it here. :)

When internet connection drops, can you login its web interface? It worked OK at the very begining, right? Maybe you can reset it of factory defaults and reconfigure it to see if it helps.