Plasma formation by microwave absorption at fundamental and second harmonic electron cyclotron resonance (ECR) has been achieved in the SHEILA flexible heliac (R0 approximately=0.19 m, Bm<or=0.2 T, (ap) approximately=0.03 m). Plasma densities up to 5*1019 m-3 (70 times the fundamental O-mode cut-off density) have been produced using a dipole antenna launching from the high field side of the resonant layer. With a low field side launch antenna (helical or linearly polarized dipole), core densities were limited to 4*1017 m-3 (3-4 times overdense). With a low field side launch either hollow, flat or centrally peaked radial density profiles were obtained, depending on the position of the resonant field layer. The hollow profiles occurred with 'on-axis' heating (resonant layer intersecting the magnetic axis) and were accompanied by an increase in the positive core potential and a reversal from a positive to negative potential in the unconfined plasma region. These phenomena indicate a reduction in the central electron confinement, which is consistent with the density clamping phenomena observed in many ECR heated devices. High field side launch produced only peaked profiles, irrespective of the resonant layer position.