Lidl Vouchers
A quick check on Lidl website shows that the vouchers are £19 all inclusive for a single flight which could normally cost up to £69. If you buy a voucher and can't find a flight you like you can get your money back! I don't think the fare is available on all routes and all flights but I didn't look into it that deeply. Basically £19 gets you a 1 way flight to somewhere on the Air Berlin network!