Finally exchanged on my flat

[Karbea]

except the guy doesn't have the full 10% so have agreed to 5%, allegedly completing on the 5th Aug...

I'm assuming you are the vendor...
Now that you have exchanged contracts all it means is that neither of you can pull out without it costing serious money. ALL the agreed money has to be paid for contracts to be completed. No-one can touch the deposit money until then so it really should not matter whether it is a 10% or 5% deposit, just that 100% of the agreed price is paid on completion.
Breathe out and don't panic!!

Thanks elbow...