Originally Posted by Heli-phile
That climb at the base leg could be trying to bleed back the speed or might be a correction for altimeter mis-setting.
Having rotated Dave's map and gone and had a good look in terrain view, there is a small ridge there, S of the A30, along to Minley Manor, Gibraltar Barracks etc. where, with light winds in the SW quadrant, I would expect there to be lift, and maybe even some significant thermic activity if it detached at the right time. Lots of dark grey surfaces, in the middle of lots of greenery, on local high ground, on a thermic day. This might be a contributory factor in the, presumably unintended, 500ft height gain on base leg, which is just downwind of where you'd expect that lift to trigger.