I fully agree with that Noel and it's how I've always operated up to now. I couldn't imagine restricting myself to a specific period and not being able to run the loco's and trains I've collected over the years.
Yes they shouldn't be too difficult to remove and there's several others it could be renumbered to so maybe it's one I could have a go at when I summon up the courage. A particular livery or depot emblem hasn't ever prevented locos from appearing out of area so I don't really have a problem running it as it is for now.
'Visiting' is a good and valid excuse - I'm with you on that. It seems like we've both got similar loco's in our collections - I've got 506 in Railfreight red-stripe livery with Thornaby Kingfisher too so I might need to borrow your excuse if you don't mind.