This paper investigates the problem of uniformly asymptotical stability (UAS) and stabilisation of planar switched linear systems using LaSalle's invariance principle of switched systems. First, we show that a common weak quadratic Lyapunov function (WQLF) is enough to assure the UAS of a switched linear system with stable modes. Then the necessary and sufficient conditions for the existence of common WQLF are obtained. Secondly, we consider the problem of uniformly asymptotical stabilisation (UASZ) of single-input planar switched linear systems. Necessary and sufficient conditions for the closed-loop system with proper feedback to share a common WQLF are presented. It is also proved that a common WQLF is enough to assure the UAS of the closed-loop system.