I want to configure BOINC so that Seti@home is the preferred project but I
want it to fall back to climateprediction.net if BOINC runs out of seti 
work units to operate on.

Thanks for your advice everyone. I didn't realise that there was a timeout 
associated with the CPDN WUs, in which case I will sit my tasks at 1:1 ratio 
and see what happens.

it's good to know that S@h WUs don't often run out these days too.

Thanks

RMC, England