Darkhawk:

 

Thanks to all your suggestions above. There's some good ideas, though I just want to focus on what position to place the extra fan at this stage. I prefer to keep the natural light coming in from the clear panels but I might install solar on the dominant side at a later date which would help to buffer the roof from the direct rays. 

Looks like setting them both to extracting is the best solution, but should I mount them side by side instead of op ends? Wondering if that would be more efficient with them both pulling in same direction instead of competing against each other.....or doesn't that matter, we are talking about 1300 cubic m of workshop space, plenty of air for them to fight over.

 

as mentioned before, i would put them in opposite ends and have one blowing in and one blowing out.

its not really about volume of air space but rather the holes in the walls/doors to let air in for the air flow the fan does. i would have to go do the calcs but wild gestimation the size hole you need is like the size of car. passive inlets need to be huge, really huge.