Thanks for your prompt reply Rich! Just joined the club and nice to know that it is alive and kicking!
Yes, the GTi's have a weak spot due to electrochemical corrosion between the aluminium wing and the steel sill, and the aluminium is the sacrificial anode and corrodes preferentially to the steel. Although an 'insulating compound' appears to exist between the two metals to minimise electrical contact, there are also bolts/screws (?) attaching the two parts and these prevent the barrier working effectively. I've had this car since new (Y-reg) so can't complain but it is an eyesore on a car which most people think is new!! As the wing is relatively easy to remove I'd rather replace than repair.
See pic below.
Thanks for your comment.