I find that actually focusing is not an issue - it's more the 'getting started' part that I have trouble with, ie. tackling serious issues/features in the code base, rather than just frittering around the edges, tidying up minor things, checking HN, and so on.

I just tried pomodoro today as an experiment based on this article, and ended up going waaay overtime on several of them. As in, an hour or more of sustained focus at a stretch. So I suspect that the paperwork and the "have to stop after 25/35/45 minutes" parts are actually impacting my productivity (by popping me out of 'the zone'), not helping it.