Danscowpie,
I may assure you that Dover, with better road connections than Manston, to Cardiff, at untimely days of the evening/night (when lesser traffic on the roads), in a tuned up 2 litre Opel Kadett GSI was circa 4 hours, that would make Chepstow circa 3.5 hours but only with, literally, motorways all the way and in a high performance car, not in a sluggish truck, and on uncongested roads!
I'd suggest more like 4.5 to 5 hours for a truck not allowing for tachograph break(s).
But I do agree with the rest of your post.
Last edited by Phileas Fogg; 2nd Mar 2011 at 22:58.