We've been to the Frisians several times and spent about 5 weeks in the Netherlands' canals & 'Schelde' estuaries. Here are a few thoughts...
In addition to the ybw forum mentioned, drop in on the SSCA Discusssion Board with your Q; there are several there who've visited N Europe
Better yet, get a copy of the SSCA CD of 10 years of Bulletins and use Adobe Reader to do selective searching on letters which describe in detail the cruising spots and issues in that region; there are a number of recent letters with very useful info and the CD is only $25.
Related to the above, the Brian Navin guide was - I found - awkward to use and poorly assembled. It's perhaps the worst guide I've had to use so far, tho' I don't fault Brian but rather the publisher. Perhaps the new edition is better (?). Consequently, I found the CD very helpful.
Visit the CA House in the Docklands area of London on the Thames. Or visit their website http://www.cruising.org.uk/
to look at the pubs they author. These are considered 'supplements' to the established guides but may well prove their worth. Moreover, consider joining one of their cruises in that area. Lots of folks there who are intimately familiar with these cruising grounds.
You're too optimistic in your view of this area being suitable for a 'quick week-long visit'. The North Sea weather
alone can abruptly change your schedule...and then there are tidal issues when visiting the Frisians. I'd suggest you take a more realistic view, perhaps by at least considering the option of leaving your boat in a convenient spot for a period and ferrying back to Britain. Weather
systems there can throw a spanner into schedules even in the midst of summer, I'm afraid.
Good luck and enjoy the canals!