Instinct Solar not recording distance correctly (since 17.50)

Since about 25 July, my Instinct Solar has stopped recording distance correctly. It is recording steps and tracking on the map correctly so the GPS seems to be functioning. It seems to be the software that is incorrectly measuring distance. This results in weird things like a 5-mile walk being recorded as 11 miles and a 4-mile walk showing up as 9.4miles.

Just to confuse things, it records the first mile of any walk perfectly. 

I've tested this out by walking 1 mile, turning round and walking back. It measured the first mile with no issues but the return leg was variously measured as between 2.4 - 4.4 miles. Making a 2-mile walk show up as anywhere between 3.4 and 5.4 miles. On the map, it clearly shows the 1-mile point correctly then puts multiple mile markers on the return leg. 

I've tried going through the FAQs, I've done a hard reset and I've manually edited the distances on each activity but nothing has fixed this. 

Unless of course I can walk a 2-minute mile without sweating and have a 6-metre stride length and the device is right after all...