My S-Type is currently having sills replaced and the whole underside re-undersealed. It was MOT`d one montha go and was not even picked up for rust. Mainly because the plastic sill covers on the S-Types actually hide the metal sills and they cannot be seen without removing the plastcc covers. That is a real easy job, but MOT inspectors cannot do it because they would have to replace them as well.
I had my sill covers removed purposely so as I knew what the score was.
I am in a very lucky position, my son in law is Sales Manager at a Main Dealer and they do all of my work. As has been quoted before, "It is not always what you know, but who you know." Good for getting the attention you need, when you need it and also benefits from family rates.
Best Wishes when you take the sills off and have a look.
Regards, John