Filling a demijohn full, nearly to the brim, is asking for trouble! Especially with mead which goes off like a rocket, or a volcano for the first few days.
Once you are confident of your recipe, or are adept at checking SGs at known volumes, and correcting for differing volumes, you can part-fill the demijohn initially to avoid the eruption of froth from the airlock, at the start of fermentation, and dilute to the full volume later.
The alternative - a simple one - is as Pargyle says. Start it off in a bucket and transfer to demijohn(s) after the first rush of fermentation subsides.
Unless using a rubber bung and airlock in good condition, there can easily be a small leak, so the generated CO2 can leak to atmosphere without going through the bubbler (now the route of higher resistance to flow). When fermentation has slowed sufficiently, the demijohn can be topped up to the neck. I often don't bother with an airlock - I simply seal the neck with clingfilm and an elastic band, pierce the film with a needle and then cover that with another large piece of clingfilm, so that gas can leak out.
Long term storage is a different matter - the DJs are fitted with safety stoppers. A small air space is then important, unless you can guarantee a very constant temperature, as the gas above the liquid will expand and contact with temperature changes, exchanging CO2 with air which is not good for the wine or mead.
RAB
