One of the things making this difficult is that the tube map depicted is a pre-2015 version. It doesn't have the Overground out of Liverpool Street (which would have filled the gap between Cockfosters and Epping), it doesn't have the trams (which would have simplified most of the southern holes) and it doesn't have TfL Rail (which would have assisted on both flanks of the map). On the positive side, the tube map depicted on the jigsaw isn't the horribly squashed version of the diagram it was about to become, so the eventually-completed version will be aesthetically more pleasing. A pre-Overground map would have had loads more white space, making it been hugely more problematic to complete, so maybe the 2014 version hits the sweet spot between practicality and complexity. I'm also pleased to see that TfL provided an accessibility-free version of the artwork, so there are no blue blobs anywhere, and wouldn't it be nice if they provided a similar easier-to-follow version of the real thing?

Something else that's not helping is that virtually all of the remaining white pieces are approximately the same shape - two opposite lugs out, two opposite lugs in. There are plenty of subtle differences, like asymmetric bumps, varying widths and lengths, uneven indentations and miscellaneous curvature, but nothing that makes the remaining pieces easy to classify. Even when all four surrounding pieces are in place and you'd think it'd be relatively straightforward to identify the one in the middle, it really isn't because there are still well over 100 to choose from. The most impressive thing is that even in a jigsaw with 1000 pieces every one is unique, and when you do get one right it fits into place with a satisfying click. A very basic rule has turned out to be "If you're not quite sure whether it fits or not, then it doesn't". And with every piece that goes in there's one fewer to check against next time, so I'm hoping the task will get sequentially easier as I continue. It doesn't feel like I've got to that stage yet.