Roundabout entry markings unclear

We were returning from a visit to Canons Ashby to go the Paisley Pear. We were on the round about and had to brake abruptly to avoid being hit by a large lorry that had joined from the A43 Brackley Bypass without giving way. We saw that the roundabout markings (showing you are joining the roundabout) from the Bypass are not clear at all, whereas the lane markings around it are very prominent, giving the impression that this is the main carriageway and the lorry drivers may not have been aware that there was a roundabout at all. This is very dangerous.