Gave up waiting for the electrician to call - and laid in the wiring myself (following the installation advice he gave on the one occasion he did turn up!) Had a bit of a battle with the 6mm2 3-core cable - but eventually beat it into submission!
Anyway - 7kw of resistive heating elements are fitted in the top of a 'clamshell'-style kiln, with ceramic fibre insulation & a commercial controller feeding a pair of 40A 'Chint' contactors in series. (The 'safety' circuit that was discussed at length isn't in there yet - but the second contactor will eventually be controlled by it).
All seems good - on switch-on nothing caught fire (!), and only parts inside the kiln got hot.
However - the contactors sometimes 'sing' or 'whistle' to themselves - by the nature of the controller the contactors are turned on & off from time to time. Sometimes they come on silently (apart from the 'thud') - whereas sometimes they whistle.
Looked on the web, and saw some comments from some kiln manufacturers, along the lines of 'they do that - don't worry'
So - is a whistling contactor a cause for concern - or is it just 'one of those things' ?