In reply to Ropeboy: Thanks. I'd spotted that picture which along with the others and the Meirionnydd CC Guide gives pretty comprehensive coverage.
Summitpost.org however has an note saying "the slabs of the upper tiers are much shorter and offer a scattering of short routes and bouldering problems, none of which have been properly recorded." I was curious about these as additional routes not covered in the guide are listed on the UKC page for Barmouth Slabs; just can't find any topos for them. Given the time I'll have available they'll be something for another day though!
David