Sorry, just repeating the TV news report re the status of the back-seater. (Always dangerous to believe anything....)
But my understanding is that the Czech seats in the L39 were supposed to be zero-zero not zero-ninety, unless you KNOW different.
The L-39 that went down was G-BZVL (formerly ES-YLB) and was owned by:
ROCKET SEAT LTD
THE SANCTUARY
HORNS CROSS, NORTHIAM
RYE
TN31 6JH
An unfortunate company name in the circumstances........