Hi guys, thanks so much for all the thoughts and advice here. Today I'm afraid the pressure got to me and I completely flipped.
I think I had some sort of a breakdown. First I rang the agent and asked what was happening, the reply to which was "I have put some calls in but not heard back yet".
To my surprise I then said, in what probably sounded like a strangled voice: "Okay, here's the situation: as of May 31 I am homeless, so this needs sorting out, otherwise I will have to reconsider my options".
She said, "Why didn't you tell us this before?" and I said, "because it was only sprung on me a week ago and because I naively thought we were completing on May 23, and because I had no idea about this top of the chain issue, I didn't see the need to tell you, but obviously this new information changes things."
I put the phone down and sobbed! I then, for some inexplicable reason, rang my bf and had a furious row with him, telling him I didn't want to move into his place temporarily (as he has offered) as we were always arguing.
Then I said "If this sale falls through I am leaving London because it is SH*T HERE!!!! and NOBODY on normal wages can afford ANYTHING, not even a CRACK DEN!!!"
Then went for my medical appt and seriously considered giving notice to my employers and going back to Australia indefinitely, and thinking "Fuk that flat... fuk my job.... fu*k everything.... I HATE it here!!!!!
Utter and total craziness. My poor bf. How on earth he puts up with me I don't know. 
Within an hour the EA was back on the phone. She told me that the top of the chain were having this survey on a place they were buying and told me in detail about it, but she said they were "adamant" they wanted to complete on May 23, and that it was No2 in the chain now causing a problem by saying they wanted May 30.
However, top of chain were "insistent" it had to happen on May 23. Which reassured me that at least the top of chain were serious about completing, and assuaged my fears that they were stalling. It helped to hear the EA outline the details of the TOC's situation.
So now I've calmed down. Tears, tantrums, but sadly no tiaras.
I won't be doing this house buying thing again in a hurry - it's nearly done me in. 