After setting the Plasma up on the workbench and removing the back panel, I noticed that it was starting perfectly each time.
What was different?
During normal operation, at each switch off, I was turning the set off at the side mounted power switch rather than leaving it on standby. On the bench, at each switch off, I was unplugging the power cord rather than leaving the 240v power connected with the back panel open. I wasn't touching the side mounted on/off switch.
I have found that as long as I don't touch the side mounted on/off switch the plasma starts first time, every time. I turn it off at the power point each night. Perhaps a fault with the power switch? Who knows, it's back to its usual self now, free of cost.