I'm fairly sure (2 days later!
) that the lines were horizontal light blue, as per the MAF Dornier.
It could have been some kind of survey flight, but I guess I would expect such flights to have been at a higher altitude, and carried out in some kind of consistent survey / search pattern, rather than a single orbit? Had it done another orbit I would have gotten a picture!
It was low enough to read the registration under the wings quite clearly, although my eyesight is rubbish and I didn't get the binoculars out in time.