Must say BOC have always changed a part full or leaking cylinder for me. Always worth giving the valve gland a squirt (on conventional cylinders) with leak detector - they quite often leak when the valves open and of corse turn 'em off when you've finished. W/A
On the odd occasions we have had a leaking cylinder - BOC have changed them. Even when we got a supposedly full one off the rack to find it was empty - we had broken the valve seal and had the reg on before we found out it was empty - we thought we might have had problems getting them to believe us - but swapped it there and then!