My understanding of the situation is as follows:
Avanti - Lots of conditions were apparently sold to take the salary as high as it is, all depots except Polmadie have four 5 day weeks and then a week of rest days (adds up to an average 4 day week), with committed Sundays this can mean working 6 days on the bounce. Sundays are flat rate, though I believe there's a booking on fee since it's overtime. I think they also have potentially big movements from booked starting time if getting a booked job from a spare turn, plus some lodging. They had a history of poaching drivers and very rarely recruited trainees, though I've heard that the DfT have told them they need to train more in-house.
Cross Country - I don't know so much about terms and conditions except that Sundays are committed and they haven't (again until recently, probably under DfT pressure) trained many people at all, historically they preferred to poach. Bear in mind that a lot of their drivers were ex-Virgin Cross Country already on high salaries. May have lodging turns as well, not totally sure.
WMT/LNWR - Sundays totally outside with certainly what was a hefty premium, London Midland tried to take the premium away and couldn't run any Sunday services for a long time as a result. Apparently there's also a lot of ex-BR terms and conditions, which has kept a lot of senior drivers from going to Avanti/Cross Country. BR terms sometimes carry cash enhancements that other companies have consolidated (depending on the specific terms). At another TOC, I believe at least two London suburban depots also with Sundays totally outside have lots of old BR financial enhancements.
Chiltern - Sundays totally outside again, I believe that historically the management culture was very good (most management were ex-BR) and turnover was minimal. They seem to be recruiting more often now, the reason for which I don't know. The only company out of the four to have any DOO work.
EMR - I don't know how out of date the ASLEF figure is because I saw a quoted figure on here that was considerably higher than the amount quoted on the Company Information pages. Newer drivers have Sundays inside, I don't know much about T&Cs.
If Chiltern and WMT sold Sundays to either make them committed or fully inside, I wouldn't be surprised if they got a salary at or near Avanti/Cross Country. GTR TL/GN showed us what the going rate is to commit to a given number of Sundays and I don't see a good DCC accepting much less.
Most of the above is gathered from what I have heard or seen (some of ti on here) and some of it may well be wrong but it's simply the understanding I have.