Hamish,
I build 90% of my rods on Harrison blanks. Unfortuantly I don't have any fly rods sitting at home just now, but i do have a specialist Barbel rod i am building for someone. You are more than welcome to come and have a look at the blank to get an idea of the quality. Its Chimera blank.
The last fly rod was a 4 piece 5 weight for someones Christmas present. They are now autoclaving on the Lorhic range of fly blanks which has thinned them down and lightened them, but i have not seen these yet. They do a 14 and a 15 foot salmon blank in this range. However, there other blanks are high quality and have a very good reputation.
Lots and lots of rod builders are using these as there own "Britsh Blanks" because you can get custom finishes and blanks built. For example, I have all my fly rods tinted with a deep blue finish for an extra £5. They will do this with any blank, the colours being black, matt, red, brown, blue and green. Its very subtle and nice
The Salmon blanks are very nice, but don't expect super modern tapers etc. They are more of a traditional blank with a very easy casting action and fish playing ability. However, they do do a "faster" 10/11 weight. I use Lathkill for ordering the blanks but you can go direct to Harrison but i found they charged more postage.
Hope thats helped a bit.