Labour would win a landslide if they had a Blairite style (minus the wars) leader rather than Corbyn. Corbyn is all very well (and I do agree with quite a few of his policies, and on balance would vote for Labour under him myself) but he's too far left to win an election.
I have voted for Labour with him as leader, and voted for him (twice) to be leader. I don't think I would do either again (well, I don't get a vote in Labour leadership elections any more anyway) though.
For Labour to win a landslide they'd need to overcome two hurdles, Scotland and Brexit. They should easily win countless marginal constituencies by this stage, but I'm not convinced they would. Brexit has caused them a huge fundamental problem - despite the party membership being reportedly strongly pro-EU, their voters aren't. Most Labour-held areas voted Leave, particularly in their heartlands. They need to win marginals off the Tories, but those areas are even more likely to have voted Leave.
For example, they "should" win Aberconwy. A Leave voting area with a pro-EU Tory MP with a small majority. I doubt they will though. (I don't know which constituency Bletchley falls in, but interestingly it looks like the Milton Keynes constituencies are similar - these are the areas Labour would need to win)