Well i'm SUPER HAPPY to report we have now exchanged (yesterday)!! After a one day delay because the seller's solicitor did not have a signed copy of the contract?

lol We are confirmed to complete on 21st June. I'm still slightly apprehensive that it's quite close to the end of June in case anything goes wrong. But by all accounts completion date is absolutely locked in even when you're in a chain (we aren't), and the only thing that the sellers
could do is back out entirely. In which case we wouldn't be caring about the stamp duty anyway! And they'd have to pay our costs. We picked that date because we didn't want too much of an overlap with the end of our rental contract. We really didn't want to be paying rent and a mortgage, plus 2x council tax for too long. But we have 3 weeks of overlap that took us to a round finish on our tenancy so that seemed fair enough. So, phew! And eek! That makes it exactly 4 weeks to exchange from when we took on our replacement solicitor. Well done her!
Yeah. By all accounts other contracts it's very normal for sellers to move into rented so this type of nonsense doesn't happen?