Thanks for your reply, i have tried the sql query
UPDATE '(PREFIX)_virtuemart_product_prices' SET 'virtuemart_shoppergroup_id'=NULL WHERE 1
but get an sql error
#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near ''myprefixhere_virtuemart_product_prices' SET 'virtuemart_shoppergroup_id'=NULL WHERE 1' at line 1
Everything else appears to be fine except the prices in the checkout.. any other idea?