Not a 331 item, but an 8-car 319 arrived in Manchester Victoria P3 at about 1255 today, and stood there for at least 15 minutes.
I've never seen a Northern double unit 319 before.
I thought it might have been the Preston stopper, but that was cancelled on RTT.
I don't think 319s have operated in service as 8-car sets while they've been with Northern - with Thameslink they operated as up to 12-car, and I think WMT have generally used them in multiple. That may have changed from today, of course.
Double 319s have operated with Northern previously, but only with the rear unit locked out. For a year or two there was an early morning double Liverpool Lime Street - Manchester Victoria, and a return working late in the evening. The point of that was, three 319s started and finished their day at Victoria (though not necessarily the same ones) but there was only room for two to stable overnight in the reversing siding. So the third was brought from Allerton. There was a similar working between Wigan and Liverpool, but it may have been one-way only, I'm not sure now.
As to the Preston stoppers, today's 1105 ex-Victoria is recorded as having arrived in P1 at Preston at 1201, but not departed again until it left P1 at 1418 to go ECS to Victoria. Other trains used P1 in between, so the ex-Victoria clearly went somewhere. Did it go to Victoria and back ECS? It may have done. Certainly, if it didn't leave Victoria until 1310 or later, it was likely to have been late at Preston.