When UK liesure was integrated into Air 2000 the seniority lists were merged dependent on the relevant size of the two companies. Practically this meant 4 Air 2000 pilots for every one Liesure pilot. Thus the impact on Air 2000 crews was minimised and it retained relative seniority for everyone. If you where half way up either list you where still half way up the "new" airline list.
No takeover is ever completely amicable but this was as good as it gets !
As for TUPE, we at Air 2 inherited Liesures 20 year pay scale and got ourselves a union!