I have managed to fix the problem. I first changed the by changing the retraction distance to 2 mm and speed to 10 mm/s. It then started to work properly. I am now using a retraction distance of 4 mm and speed of 15 mm, no stringing at these settings so I'm happy . I have not tested it back to the original settings of 6.5 mm and 25 mm/s (whilst it works I will not fiddle with it!). I am not sure if it was the setting change that did it (can't do 6.5 mm and 25 mm/s) , or some how playing with the settings fixed something (unjammed a cog?) and it can work at full functionality? It works for now and I keep those settings, unless otherwise advised.