Ok, have now got the T3 to power up... sometimes!
I've found 2 of the FFC cables (very thin ribbons) to be damaged.
One was an IDE connector - now have HDD but no optical drive.
The other is the connector cable from the PC unit to the front panel - this is the one causing most problems as it seems to have some control over the power up process.
Can I by these cables as spares from anywhere??