Bristol Brewers

Might be a bit extravagent but I'm planning on making a batch of Mild and splitting it in two at the fermentation stage to litterally pitch two yeasts against each other a nottingham (dry) and WLP037 (liquid) to see how they differ. I do have two fermenation bins but both are 30L and I'm a bit concerned there will be too much space at the top of the bins with just 10-15 litres in both. Will this matter too much?

I'm wondering if mashing to a higher OG in my 5 gallon set up, then watering it down at fermentation stage might do the job? Any other suggestions?

The fermentation will give off c02 which will blanket your beer. As long as you don't leave it sitting around for a long time post-fermentation in a bucket then you'll be fine.

Excellent, and cheers for the quick response.