BigChange Senior Software Engineer interview questions

Three stages - first over Teams as a casual chat with the head of dept, second more technical over Teams with the same person again plus my would-be manager. Was told I passed the second stage and was invited in to see the COO and CTO (the CEO also popped) as a final stage, spent 2 hours having a great chat with them and getting shown round the offices. They left me hanging over a bank holiday weekend, eventually called me on the Wednesday to say they weren't going to make an offer as there were "some concerns on the technical side" from the second interview - the one which I was told I had passed and so spent hours travelling and chatting with their very expensive senior management team. I never did find out what the technical concerns were - looking at the listed salaries on here I think it was that they didn't want to pay what I was asking. Really poor feedback, and bad form to say they were not offering based on a loose reason from an interview stage I had been told I had passed. Plus all the subsequent wasted time, it left a very sour taste.