Hi. This is my first post, so I hope I get this right!
(All the following is assuming the nochex APC contrib, latest version)
I had the same problem trying to get a "Declined" message to test that functionality... It seems to be the number format in the Nochex form submission, not the currency format in the Admin form.
In checkout_nochex.php, around line 242, you'll find:
tep_draw_hidden_field('amount', number_format($order->info['total'] * $currencies->currencies['GBP']['value'], $currencies->currencies['GBP']['decimal_places']))
Replace that with
tep_draw_hidden_field('amount', number_format($order->info['total'] * $currencies->currencies['GBP']['value'], $currencies->currencies['GBP']['decimal_places'], '.', ''))
This changes the result of the PHP "number_format" function to use "." as the decimal and nothing as the thousands separator.
For others with the same issue on other payment systems, check the PHP doc at this page
As always, backup and test and don't blame me!
Best of luck,
Ian