Chain transactions are notorious for problems like this as everyone has to be ready before proceeding and you are dependant on people you, your legal advisors and agents are completely unconnected with.
The best people to try and find stuff like this out are the agents as they have an incentive to get things through to get their commission.
If there is no direct contact between the different parties (buyers and sellers) in the chain contact the agents on your sale and ask them if they could make enquiries down the chain in order to find out what the hold up is.
There is no time period for exchange unless it has been previously specified by a seller eg a reservation period on new build properties. Unless otherwise agreed, both sides can exchange or pull out when they like.
It is possible, although frustrating, that the people at the bottom paid by cheque. They usually take 5 working days to clear but can be put on a 3 day special clearance.
As for the 8 days there could be a number of reasons such as notice periods etc... it is more than likely the time that is needed to request mortgage monies from their lender. If they do not get it in the completion date then they would be in breach of contract and serviced with notice (and a notice fee as the per the contract eg �100 plus VAT) and charged interest on the whole purchase price for every day is does not complete.
Service of notice will then tend to pass up the chain which makes everyone suffer and will delay completion further, incur costs etc...
Their solicitors are probably making sure they get the mortgage monies in in god time for completion to avoid this happening.
Hope this helps!