Thanks guys for your comments, all very much appreciated.
Chris - Wingnuts have improved their wicker seat, it no longer has the extra thickness of plastic at the back of the seat covering the crossed open weave section, so drilling it out was very easy as the plastic between the weave was only very thin, made life a lot easier.
Bo - the Tacho drive is a very small spring, it is 0.5mm in diameter and 20mm long, easy to fit and very flexible, also the correct colour.
Des.