Dear All,
Many thanks for the responses; they're really appreciated.
Very good point made - of course, add the 100% O2 first, then top off; my mind had fixated on the heliair mix method.
Mixaddict - I promise you we are religious about O2 cleaning and testing, Gold Card members at G-Squared in Burgess hill in fact!!!
Clearly we need to double check though.
Thanks again to all and have a great (safe diving) weekend.
Pete