We've found the source of the problem with this - there was a corrupted node on our map database. We've implemented a fix and this should take effect in the next few days.
Sorry Matthew, we misunderstood this issue - we can see the problem now. It looks like something has gone wrong with the map tile rendering at a point along the A94 just north of Dundee, which has resulted in an unexplained diversion. We're looking into it and will get it fixed as soon as possible. We'll post back here when we have an update.
Hard to know why that happened Matthew. Try reducing the distance between the points you plot.
For some reason the UK road, the A94 goes on for ever around the world in a straight line...