Having mapped many of the streets of Teignmouth today I hadn't realised I'd performed a miracle - see map.
There are possibly three explanations:
1) my gps was giving me false readings
I checked it against Garmins GB map and the track more or less was spot on.
2) the tide was out
Could be, but it has been out ever since I moved to Devon (35 yrs)
3) the osm coastal data is not 100% accurate or up to date.