US Tax liability for a UK company selling to the US

We are a UK Ltd company who have been selling goods to many different business organisations in the US for the last 17 years. This post ( newcomer link: http://talk.uk-yankee.com/index.php?topic=67170.0 [nonactive] ) suggests that we should have registered with the IRS right at the start and should have been submitting annual tax returns to the IRS ever since.

We have NEVER been asked for an EIN or TIN by any of the organisations which we've sold to over the years. We have also always been paid in full for the goods which they've bought - none of the companies have ever withheld 28% tax from our invoices.

For various reasons we now need to apply for an EIN number  and I was spooked by the part of the form which stated that we'll be asked to also complete 1120F  Income Tax Returns for each year since we started trading with the US. If we go ahead with the application I'm worried that we may suddenly be stung for 28% tax on all the sales we've made to the US over the last 17 years. All our profits are declared in the UK so we've paid full corporation tax on these each year.

Does anyone know if I'm missing something here. I really hope that I am!

Many thanks.