Don't know what made me think to do it but it really focuses the mind, attaches it like a barnacle to the breath. Nice practice, how many times would I have chanted it I wonder 4-500 times (Just worked it out, close to 600 breathes in Primary)? Practice ended up taking around 90 minutes.
mantras that work well for this would include
Om namo narayanaya
Om hrim namassivaya
Looking forward to trying it on 2nd series
One amusing moment, actually not so amusing at the time. It's getting colder and I couldn't get a good sweat up, just that clammy sweat that's almost sticky. Only managed to get my arms half way through in garbha pindasana, managed to roll but when I came up for Kukkutasana I could only hold it for a breath before I toppled forward, arms were so stuck solid that I ended head first into the mat, just managed to get my forehead down, ouch. Now that's why you need a manduka.