Possibly the worst track ever

Wow, that looks pretty bad!

How should it look like, and how does the rest of the run look?

Rest is pretty ok .I did swap to GPS+GALILEO part way . Mostly the track should be on the White lines except for some short excursions into likely off shoots

That offshoot at the top into the trees is real as is the short trip by the 15 marker

actual track

Looks like there’s a few long straight lines in the false parts of the track, which might suggest that your watch has lost it’s GPS fix, drifted and then drawn a straight line when fix is re-established.

Have a suspicion that pylons nearby didn't help

I have seen exactly this on multiple GPS watches over the years under some pylons over a popular local nature reserve near me.  The worst I saw was a friend with a Suunto Ambit Vertical, where it literally 'jumped' over 2 miles near the pylons.  Odd thing is it only impacted runs infrequently in that area.  So unless you see this all the time, I'd just chalk it up to one of those power pylon anomalies.

That was my thoughts

There's another area north of me (near a place called Govilion) with pylons where I do get similar, and as you say in that area it's a random effect and not always seen. I don't normally take the turn at the 13 marker and normally carry on up with this one - so thats probably why I havent seen it here before

Any electrical signal can disturb the GPS! I had this with my Fenix 3 when I passed a guy starting a chain saw. The watch lost the position and didn't find it back until I reached home. Never happened again, so it clearly was not a fault of the watch!