pint6,
From our experiences which are similar to yours (we were due to close originally end of Jan but due to delays on our vendors new property we are looking at end of May!) if you have signed contracts from both parties then each of you are legally bound to complete the transaction. The actual close date on the contract is pretty meaningless and if the vendors want to extend the closing date then they pretty much can regardless of whether you are in agreement. Likewise you could do the same to them, however the contract probably stipulates that you would be due interest to them but this is notoriously difficult to claim. Most contracts state that any interest claims will not delay the closing of the sale.
There is the option for either party to issue a completion notice, however this is seen as a hostile move. The notice to complete gives the party the notice is served on 28 days to close the sale. After this time the party that issued the notice is legally entitled to pull out of the deal charging interest on any monies owed. If the vendor wants the sale to go ahead it is normally at this point they would find temporary accomodation. This notice is normally issued to the buyers if they are delaying the sale but it can also be issued the opposite way.
It seems to me, from our experiences, that the vendor has the control in these situations. We aren't in a chain (FTB) but I can't imagine the situation these delays would cause if the chain is long.
If you have both signed contracts and the vendors did pull out then you would be able to take them to court for breach of contract and you would be successful in this action.
Your solicitor should be able to advise you, as ours did, on all your options going forward.