kengr

Not that hard to get it back out, depending if you needed to do it stealthily or not, and how big said ship was. If no bigger than a TARDIS, quite easy. If you don’t need to be stealthy, could wrap it in plastic (or equivalent, put sand around it, then cheap concrete over it. Later, just jack-hammer the concrete. Don’t need an access stair (rather same as your bury it in a peat bog idea doesn’t need access stairs).

I wonder. How long have self-storage rental places been around?

The key to having it be in a building would be the administrative/corporate structure that does the upkeep and continuity of the building itself. I suppose it’s rather like stories about vampires, where they run corporations for 100’s of years to keep their wealth/treasure/keepsakes. In those stories, they pass the directorship on to a “nephew” or “cousin,” who is actually the same vampire. I suppose our time traveler could do the same, show up at the annual corporate board meetings once/year as “a different person.” Even without that, banks and trust companies can run a building for you for 100’s of years. I suspect that in UK/Europe, there probably are such things, with or without vampires or time travellers.