[raesene9]

From what I've read of it the mauritian government didn't really want to sell but the British implied strongly that it would be a pre-requisite for them gaining their independence...

To quote a british official (from https://en.wikipedia.org/wiki/Chagos_Archipelago_sovereignty...)

"in theory, there were a number of possibilities. The Premier and his colleagues could return to Mauritius either with Independence or without it. On the Defence point, Diego Garcia could either be detached by order in Council or with the agreement of the Premier and his colleagues. The best solution of all might be Independence and detachment by agreement, although he could not of course commit the Colonial Secretary at this point. "