Had a chance to have another look at this again tonight,
If you remember I was having trouble starting the shaper, the contactor relay would just rattle for ages before locking on. I wasn't sure if it was the starter or my 3ph rotary converter. The shaper motor is 3HP and the rotary is also 3HP. Not a good combination as rotaries should always be larger than the motor you want to start due to a motors high surge current when spinning up to speed.
However, I can report that the "underrated" rotary converter starts the 3HP shaper motor just fine with not a murmur of any complaint ....

Phew, thank goodness for that !!
The problem seems to be the contactor itself, it's actuated by a remote switch higher up on the machine. I took the cover of the contactor and started it directly by pressing the coil switch. No Problems at all, the motor spun up instantly in all four gears engaged. i.e under load.
After starting the machine like this for a few times this evening, doing some shaper work, I tried the remote start button and it seems to have got a lot better. Not instant, but nearly with hardly any rattling of the contactor. Towards the end of the evening the contactor started to behave altogether.
So my conclusion is that the machine may have been standing for a while and some corrosion had probably started to build up on the contacts?