Great advice – the tape idea is excellent – my usual problem is wandering slightly off course when hacksawing so that will help me enormously.
A packing piece it is then; I have some suitable marine ply so will make up a block and epoxy & varnish it against UV as the base for the new step so it should last a long time or I might just take a hunt round the kitchen for some Delrin when the missus isnt looking.
Yes I have the correct foot – offering up against the old one it looks perfect: its one of the metal cast ones that Selden still supply for these older Proctor masts and fits the newer composite channel beautifully. Getting the old foot out looks like its going to be fun though as it looks well seized. I may be able to organise it so the cut is slightly above the inner part of the old plug, failing that it will be lots of wd40 and patience.