Absolutely !

It seems that the mandatory postcode field has all kinds of non-postcode stuff in it !

For anyone else with this issue the original error was:

Invalid length parameter -2 to SUBSTRING. Negative integers are not allowed.

Changing the query to

SELECT DISTINCT

left(c.addr_5, CHAR_LENGTH(c.addr_5)-3) as trimmed

FROM vecusts c

where CHAR_LENGTH(c.addr_5) > 3

solves this issue and now only leaves what is left to be validated.

Thank you Helen J