OK the problem is caused by the upgraded version of Mappoint which tries to be 'clever' and map the addresses to 'street level' not just postcodes (as before). This leads to the incorrect plotting of addresses even if the postcode is not recognised (i.e. it defaults to use street address rather than postcode if postcode is not recognised).
DON'T DO THIS IF YOU NEED PINPOINT ACCURACY!

To overcome this, when the columns of data are displayed select 'other data' in the drop down lists for everything other than postcode.

It was due to the talents of Simon who worked this out.

Thank you Simon.

We love you