Hmm good shout Stu, it was certainly cold and icy this morning when it switched itself off after 4 miles and the other occasion. Hopefully that's all it is i can live with that if it's just very occasionally. Might make sure my battery is well topped in the cold in future see if that cures it, I like the F55 easy to use and this is the first issue i've had with it.
Edit- Just noticed my whole run has been tracked not just the 4 miles but the data from 4 onwards isn't there. My watch was definitely off after 4 miles. Bizarre.