Strake, ideally the best option would have been to cancel the flights at loss of deposits and claimed back on insurance. even if you know there might be a need to cancel, most insurance companies are happy to let you 'hold onto the booking' provided that when it comes time to cancel the booking it does not cost any more to cancel the booking than the original cost of cancellation when the first instance arises - eg £150 to cancel when you are made aware of need to possible cancel in say june, but as long as they cancel say in sept when they can still cancel for £150, insurances are usually fine. as a travel agent, this is what i would have advised, and i'm sure most would do the same too. i would not advise anyone to pay a balance for a holiday/flight that they know that they are not going to travel on.
dtaylor, totally agree with you about the chargeback. a chargeback situation can only be done when a provider fails to provide the service or goods that was paid for - (eg buy a TV from online electrical retailer and it never arrives). in this situation, globespan provided a seat/s as requested, it was then the clients choice to cancel the booking. therefore globespan has provided what was requested.
Personally, i would go down the line of a second, police letter to Globespan explaining the circumstances, with maybe a covering letter from the specialist.