Relisting is a costly option for everyone. We were recently on the vendor side of this equation as our buyers were being very slow. As much as it hurts dragging the sale on, you're already quite heavily invested in the process from a legal and other fees perspective (potentially, at least) plus you've invested time, possibly months, and corresponding mortgage payments etc only to have to potentially wait for another buyer who might be equally as slow to move. They'll never recoup the 'lost' time and mortgage payments unless you've lowballed them/screwed them down on price. Their EA won't want them pulling out either as it means continued effort on their part remarketing the place and so on.
Until/unless there's an explicit threat of relisting coupled with a deadline, I wouldn't personally worry about it. House sales take time, everyone knows this, it'll very likely be OK. Don't worry!