Which Arkel pannier to you have? I have a Tricross, using a Sherpa rack and a T-28 pannier. There is no heel strike and I don't have it on the T-42 neither. I checked the geometry, and the Tricross chainstay length is 440mm vs. 405mm on the Langster. Is it severe heel strike?

I think the Cold Springs/Sherpa racks by design allow more clearance from heel strike than the Red Rock/White Rock. Call or email OMM and ask about some advice. They have excellent customer service and be more than happy to help you out.