Indonesia is a big country, a really big country, and that is particularly obvious when you are from the UK, which is vastly smaller than just one of the Idonesia islands like Sumatra. Getting around this leviathon can take time, and if like me you didn't bother planning ahead getting a longer visa from an embassy beforehand, you will probably find the 30 days visa-on-arrival (VOA) simply is not long enough to see it all. You are in for some fun extending your VOA during your stay in Indonesia. Let's just say, the adminstrative system can be rather interesting.
After googling for some time how to extend my visa in Indonesia on Flores, I was pleasently surprised to find I had got rather lucky. Not only was there a relatively new immigration office ('Kantor Immigrasi') in Lubuanbajo (the little fishing town that is the jumping off point for trips to see the famous dragons of Komodo), my hotel the (also newly opened!) "L Bajo Hotel" was right next door (this proved incredibly useful, not only because the manager was super friendly and willing to out of his way to be our sponsor, form translator and to help us with photocopies and printouts, but because the staff at the immigrasi seem to like to make you work for your visa and going back and forth is part of the deal. Many times there, we saw couples getting taxis to and from their hotel, and counted our lucky stars that our hotel was within 5 minutes walking distance.)
Well like I say as of writing the L Bajo Hotel is relatively new and only opened its doors in Jan, 2015, so it doesn't really appear on Google maps, nevertheless if you find the nearby Pagi hotel you can just walk up the road to the L Bajo, continue onward past the "Komodo National Park" (big sign outside) office, before finally taking a right going past "Dhiva hair salon", something like this:
They are open 8am-12noon, 13h-16h (I would go early rather than near lunch as things can take longer than expected and you don't want to be waiting there over the lunch hour).
Even for a VOA extension we were told you need a sponsor. This can normally be arranged with the manager of your hotel (at least the friendly chap at the L Bajo helped us out). They will need to provide you with a photocopy of their KTP identification card, fill in a section of the extension form (included details such as their income, and how they know you --- "friend" usually suffices). You will have to take this form to the post office (this is on the main strip near the restaurants so you can't miss it) and buy a stamp for 6,000Rp. After adding this stamp to given section on the form, your sponsor needs to sign half over it, half on the page (i.e. prove they signed it after the stamp was added). You should have a second form too that needs to be filled in using black ink and in block capitals (this was mostly personal info and nothing for the sponsor if I recall correctly, although you will need his phone/email). You need photocopies of your passport photo page and the page in your passport with your existing VOA. Make sure these are on sepate printed pieces of paper (not just on different sides of a single page) otherwise they will make you reprint, shrug... Last but not least you need your exit ticket booked and printed (clearly showing balance paid). We booked a cheap flight with AirAsia from Jakarta to Malaysian Borneo, but I'm not sure if other types of transport (train/ferry) are just as acceptable (I don't see why not, but can anyone confirm?)
You will need to dress "smartly" each time you visit the immigration office (something of a power trip or what? I got sent back for three-quarter length shorts, so they really do take this seriously unfortunately), for me jeans a t-shirt and converse were of sufficient smartness to qualify (oh, how I wished the immigration office had A/C that day). No shorts, no sandals, no flip-flops etc.
After you have all this material you can take it along with your passport (which they'll keep during the period your application is being processed) to the office, and they'll probably take around three days to process everything. It's best not to cut things too fine however, they are notorious for sometimes taking longer than this (computer systems down, national holidays etc).
In theory that should be the hardwork done. In 3 days you can go collect your extended VOA. At the time of writing it cost 355,000Rp. You don't need to take a passport photo as they have cameras and fingerprint readers there. They'll keep you waiting for an hour or so, take you to a small room, take your photo and fingerprints, ask you some questions ("what is your purpose in Indonesia?", "occupation?", "do you speak Bahasa?", "when do you leave and to where?") and comment on how nice your trousers are ("grrr"), you'll wait another hour (this was with an empty waiting room bar us! I think they were just making us sweat, quite literally for that matter), then finally you'll be hopefully presented with your shiny new VOA extension stamp! hurrah.