In reply to Reach>Talent:
Although you don't have hills to run on what about bridleways/footpaths/parks etc anything with an uneven surface will help, beach / gravel.
When stood up try standing on 1 foot without wobbling, then a slight bend in knee, or throwing and catching a tennis ball against the wall.
Also wobble board and then wobble board with the tennis ball, heel drops on the stairs as well.
Rich