Kestrel was indeed the Airtours International call sign (VZ/AIH)
Aspro Holidays and their inhouse airline Inter European Airways were acquired - They also acquired Danish charter airline Premiair (was Conair and Scanair merged)
Airtours later renamed themselves MyTravel (MT/MYT) Kestrel was still their call sign
(I had retired by then)
A merger in 2007 with My Travel Group and the Thomas Cook Group (TCX) formerly JMC Air (MT callsign JMC) but even today flight numbers from UK airports still use both MT and TCX for flight numbers -
why? dunno....
Kestrel callsign has just been replaced with Thomas Cook
I have no idea who uses Tourjet now (if at all)