A bit of an update - I've tried the original bolts and it's not an improvement.
Next I'm going to fit a different drive plate as I have one - thought occurs to me that the original rubber centre section might be the cause of pushing things off centre. As I have a spare one that's a no cost option.
Second option is to try a different drive casing / hub unit, last option as it's the most expensive - fit new axle & nut.
Update: I have a drive untit/hub on it's way as insurance - found what might be a reasonable condition one for less than £24 inc postage.