Yes, it was. I feel a schoolboy error moment coming on....
Not your fault at all, it's poor design on Paterson's part.
This is caused because the spiral goes up the column as you invert the tank - there's enough space on the column of a Universal tank for a 120 spiral to go up by quite a bit. As a result, the 500mL of developer no longer fully covers the whole spiral, and you get that under-development on one side of the film.
On older Paterson tanks they supplied a little white stopper to ensure that spirals didn't move, but they omitted them in the Super System 4 range. As a result, every few months we get someone (including myself, a while back) posting with the same problem.
There are a few different solutions - the one that I do is to emulate the white stopper by getting a thick rubber band, putting it around the column (after the spiral has gone on), so the spiral cannot move up any further.
The other alternatives is to put an empty spiral to fill up the column space, so your spiral physically cannot move up further, although this means more (somewhat unnecessary) cleaning, or just to use more developer (more costly, a tad unnecessary as well) in the tank.
Yours hasn't fared too badly - my agitation technique is quite vigorous so when it happened to me, the roll moved quite far up the tank, causing this:
http://www.talkphotography.co.uk/forums/showthread.php?t=379354