As far as I can recall, the instruction to marry again in the UK came from the British embassy. I assure you we wouldn't have done if it wasn't necessary at the time. Perhaps the rules have changed since.
Anyway, an update on this. I paid 5 million to a lawyer in Indonesia to track down the divorce certificate and mail the original to me, which is a little bit cheaper than hauling myself over there and slogging around the streets. Getting hold of the certificate from the KUA office, once he had found the right place, was apparently very easy and straightforward - he has shown me a pdf copy by email.
However, now he is demanding another million just to mail the original (which I need) because he says he can't afford the postage after a run of bad luck(!) - even though our express agreement in writing included the cost of mailing. Unbelievable. I didn't pay 50% upfront and 50% later as it's not a big amount and I thought for once it would be nice to put some faith in people. Ho ho. Also, his office has a website and looks like a decent outfit.
I wonder whether this kind of thing is normal when dealing with lawyers in Indonesia? A request for additional payment by way of baksheesh...?