Glad to be of help. It's a great book called Architecture of Death by George Nash. According to the back it covers "the 100 chambered tombs within Wales that have significant remains" and it also lists other possibles and destroyed sites.
I've found it really helpful as it has descriptions pictures and plans for all the sites, grouped by area, so you can see the stylistic differences between the regions.
Downside - it was quite pricey!
We'll probably go and see Sling on our next Snowdonia trip, but again need good weather.
sam
