Just to clarify....

When we send all out paperwork off, including the third country certificate to the USDA vet to be stamped, is there a time limit after that in which the dogs have to travel? I remember reading something about 10 days- and also 4 months?

There is, but I can't remember what it is...

Found it!!

From the DEFRA site http://www.defra.gov.uk/animalh/quarantine/pets/procedures/support-info/documents.htm

Validity of the certificate

You need to be aware of the following. The certificate will not become valid for entry to the UK for 6 calendar months from the blood sample date shown in section V. However, it is only valid for entry to the EU and subsequent travel within the EU for 4 months from the date it was signed or endorsed, or until the “valid until” date shown in Part IV, whichever is earlier. You should therefore delay getting the certificate until a month or two before you travel to ensure that it will still be valid when you enter the UK.