The fact that it looks as if I did some walking on water isn't necessarily wrong, because the location of the pond is only what I have edited on the the OpenStreetMap background map, working as well as I could from a Google satellite image, taken when the water was largely obscured by the tree canopy. What I was hoping was to use the track recorded as a way of improving the pond and other features on OpenStreetMap, but as it stands, this is not going to be much use.
I was expecting it to struggle, because even in winter, the trees are likely to interfere with the GPS signal, but afterwards it occurred to me that if I repeated the route a few times, there would be some averaging process which might converge on the truth. Googling,I find there is, which is fairly technical, but I'll give it a go.
Meanwhile, if anyone else is interested in using apps like Strava to help improve local maps, get in touch!