I also have searched many times for clips - but strangely enough I've just come across a full 3 minute clip of episode 6 of Seal at the BBC's site - which says its been there since 2009! & will be there for another 4 months.

Its to do with the Postgate documentary from that time - but isn't just the few seconds used in that - but is just over 3 minutes long ( edited from different parts of the episode ).

If you google -

seal of neptune postgate

it comes up top of the list.
I was tempted not to watch & wait until December for the DVD but I couldn't resist !
- & most wonderful it is too!

I can't believe we didn't find this earlier & I wonder if perhaps before it wasn't correctly worded at the BBC site & so didn't turn up in google searches ( ? )