Many thanks for the good advice. Success!
Firstly, I remembered that the 2-4-1 was actually made on my wife's companion voucher and account. Logged in as wife and lo and behold, MMB revealed a functional 'change my flights' option. Job done!
Children's flights made on my account - logged in as me and changes made on each. Job done again!
There was no charge for any of the changes.
Bizarrely, our household account activity shows re-deposits and redemptions of miles for each of the three sets of changes, but the redeposits were greater than the redemptions to the tune of about 1.5k miles. Perhaps this represents tax/fuel charge reductions? Anyway pleased with the changes and in-pocket too.
Shame that the BAEC staff offered such incorrect telephone advice, all be it very politely!
Mark
C