It is easy [by looking in my book] to confirm the starting sequence was
3421 or for a Push Back it was 3 then 2 followed by 4 then 1, after pushback was complete
Now the hard part is to remember why, and perhaps it was because that is the way in BOAC/BA we had always started engines, but on previous aircraft I do remember there being a reason such as brake pressure or electrics, which was not the case on Concorde. However I seem to remember that the hydraulic pump layout on pre production Concordes was not always the same as the airline version, so this might have had some influence
However by starting 3 then 4 first it did allow engine start to commence with the passenger finger still in place. Now unlike the French the flying control checks were carried out by a pilot as engines were being started and it started with "Blue" being selected which was sourced from 3 and 4 engines.
Now I think this was only because 3 and 4 were the first engines to be started rather than the reason, but it was handy to speed things up.
Now I have heard a rumour you understand that sometimes when things were running late the 3 and 2 engines would be started at the same time, but you have to understand this is only a rumour
