I only managed to get to the site for the first time today. Too many other projects have cropped up unexpectedly. I was finally able to terminate the cable and can confirm that over a TDR measured length of 22m I'm getting a delay skew of less than 7ns for each pair.
A long bit error rate test shows zero errors too (107 random 1518 byte packets at a link speed of 1GBps). The link happily runs at gigabit speeds which is all it needs to do for the installed Unifi UAP-AC-IW access point.
I still don't know what the cable is but there's about a metre and a half of it inside trunking that I can get to to see what's written on it. I'm back in the morning so will have a look. It's definitely for structured cabling use, I just don't know what use
I haven't gone searching for the electrician's card yet.