Thanks to all those who contributed to this thread. I am now at Departure Gate 86 at Arlanda waiting for my BA flight to Heathrow. Everything went very smoothly at Bangkok and there was no problem having my suitcase tagged through to Heathrow and I was also issued a boarding pass for Stockholm to London (belt and braces as I had already checked in on my phone and had a mobile boarding pass).

The flight from Bangkok was on time and after a four hour transit at Doha, my Qatar flight left more or less on schedule and touched down at Arlanda just five minutes behind schedule at 13:25. We were on Gate F36 Terminal 5 shortly after, and I was amongst the first off the flight at 13:37 and made good time along the walkway to the Immigration point where I had a choice of going through immigration (no queue at all) and briskly walking from terminal 5 to Terminal 2 or checking out the bus option. As the whole point of this thread is about getting to and fro terminals 5 and 2 airside, I decided on the latter option.

Gate F1 is the bus gate, and the signage clearly tells you to press the button and wait no more than 10 minutes for a bus to terminal 2. I pressed the button at 13:43 and a green light lit up around the button - reassuring! After three or four minutes a security officer emerged from a nearby room and requested me to put my carry on baggage through security and did a body search as there is no scanner here. Two minutes later the bus arrived - a massive bendy bus just for me, and I was on my way again by 13:50. I was dropped at Door 70 Terminal 2 and took the lift up to Gates 86 and 87. I was seated at departure gate 86 at 13:57.

The only downside, as has been pointed out earlier, is no access to lounge or restaurants, but that's a relatively small price to pay to save a 30 minutes fast walk. Today is Sunday, and no doubt it might have taken a while longer had I landed at a busy period, but unless one is very unlucky 1 hour 40 minutes is plenty of time to transit Arlanda from Terminal 5 to Terminal 2 (and I assume vice-versa)