After I've spent all day reading and re-reading my logs I have now noted that the lines regarding tax incorrect were 'Line-item-details skipped.'

Further down the same log file I have this info

Code:
Dec-26-2009 08:51:41
before_process - EC-5
resultset:
Array
(
    [TIMESTAMP] => 2009-12-26T08:51:41Z
    [CORRELATIONID] => *********
    [ACK] => Failure
    [VERSION] => 3.2
    [BUILD] => 1105502
    [L_ERRORCODE0] => 10417
    [L_SHORTMESSAGE0] => Transaction cannot complete.
    [L_LONGMESSAGE0] => The transaction cannot complete successfully.  Instruct the customer to use an alternative payment method.
    [L_SEVERITYCODE0] => Error
    [CURL_ERRORS] => 
)

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Having already read many posts relating to 10417 errors being Customer/Paypal related I think my problems is simply someone trying it on.