According to the manual there is an option to change this when the router is first used but I cannot find anywhere to change it now.

The 5 minute logout is a major pain and I need to change it to something like 15 minutes.

Is there a way to do it?

I dont know which manual you referring ... as far i know are 2 user manuals ... V1 & V2 (but have not check them to confirm if are the same) ... both to my opinion are out of date due to new features after 3.6.3 firmware ... how ever i am using the beta r34169 htc fix (3.6.3 RC1)from UK support pages and it has option for off, 1,3,5 & 10 min ... so seems better than the 5 minutes is allowing your current firmware

I am using the plain 2.6.3. Where exactly did you find the options in the menu tree?

Perhaps you mean the 3.6.3 firmware as 2.6.3 is not exist
Here a screenshot of the auto log for 10min or off (stay login all times)
Note the coming new 3.6.4 firmware under management settings can set permanent off from there as well

Thank you. I installed 3.6.3 a few days ago (sorry for the typo) and did not spot it staring me in the face! I was hunting for it in the Management section.

The way it is working now is in fact just fine for me as I do not want to turn it off permanently. I do sometimes log into the router remotely and then I would probably want it to log out automatically.

Maybe the Management section should have two options - one for local and an other for remote.

Thanks again.

You can just set it to off ... work the time you need (that what i do) and then down left of this red column has a gray sign out button (same side down page)
However the new firmware 3.6.4 i think should have more option in management section but at the screenshot i provided should have (to my opinion) longer timing than 10 min ... something like 10, 30, 60 min, up to day 24hrs