20 Kilos Standard allowance + 10 kilos extra with First Choice for diving gear if you ring at least 4 days before travelling on
0871 664 0144: First Choice - Information on Luggage.
They send you out a letter to show the check in staff. You need a qual card to show at check in, tanks must be emptied to 15 PSI.
Done this 3 times in last 18 months (europe & long haul) and never had a problem. I also get the missus to take her PADI card to claim her 10 kilos as well!
