Definition: Given a travel related situation, and given a specific question about the situation, summarize the question into a title
Input: I am a UK national and I am travelling to Indonesia for 35 days. Can I get a visa on arrival and then extend it for the required days?

I am reading conflicting information on sites. Some say that visas that are obtained on arrival cannot be extended. Others say it is fine. Not sure what to trust.

I don't have lots of time before I leave (13th of May) so will I have time to apply for one online?

UPDATE AFTER MY TRIP: 

Here is the process that we went through whilst in Bali.


When you land, before you get your luggage you will have to pass through Immigration. There will most likely be a massive queue for the FREE 30 Day Visa - you don't want this one - you want to pay for a 30 day visa so that you can extend it later. There will be little desks at which you can pay for it. The price is about $35. They give you a little piece of paper that says Visa on it and they point you to the immigration desk. A member of staff there will question your stay (standard procedure: where yourt hotel is, what you are doing here etc...). They will stamp your passport and let you though. 
Two weeks into our stay we went to the Immigration Office located in Jimbaran. On your first day there you will need to bring with you the following:


  2 copies of the ID page in your passport
  
  2 copies of the visa page in your passport
  
  Your passport
  
  Confirmations of hotels/flights (not 100% neccessary but good to have)


At the desk you first see, ask for a Visa Extension form. Fill it out.

There is a machine which hands out tickets. Ask a member of staff which button to press as the tickets vary depending on what you're there for and most of the options are coded with abbreviations that no-one understands.

You get your ticket and wait to be called up. They will take your documents and passport. They will then give you a day about A WEEK LATER to come back. KEEP THE DOCUMENTS GIVEN TO YOU SAFE! YOU ARE NOW PASSPORTLESS AND THIS PIECE OF PAPER IS ALL YOU GOT. 
You come back on the given date. On this day they will ask for payment (355,000 IDR) and they will take your photo and fingerprints. You once again need a ticket and so on... You will be given some documents which get passed around the different officials you will encounter and they get stamped a couple of times. You will once again be given a return date about a week later.
Final day. You come to collect your passport. You go to a little side window (ask a member of staff: just say you are here to collect your passport and they will point you in the right direction). You give the forms you have. They just tell you to sit down. At this point you just need to wait and carefully listen out for your name; they are prone to mispronunciations! They will give you your passport and in the visa page you will see a new stamp that says your stay is valid until a much later date. Done!


Here is blog post I found that explains a similar process

Output:
Can I get my tourist visa on arrival at Indonesia and then extend it?