I emailed them for replacement wheels for those with the traction tires. Had a response quickly and ordered them successfully.
Here's the direct email point of contact:
arvids.breikss@hornby.us
Yes, Arvids is a super-nice guy (I've dealt with him through email and in-person).
But I also had the same problem with my initial email contact. That sends the email to the Hornby mothership in Europe. It takes them about a week to forward the request to Hornby US. Then Arvids responds very quickly. So the above email address is the best and quickest way to contact Arnold/Hornby in US.