Ollie,
I believe 400mm is average max zoom required for airshows considering 1.5X crop of a DSLR. Approx 2/5 use Sigma 50-500 or 170-500, try to avoid 'cheap' lenses as their optical quality is exactly that, instead buy thrifty on the used market be patient and haggle!
If you sold your Z2 privately it would go for much more, remember Jessops want to sell it on at a profit.
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