I have just started the process of starting a pet passport on one of our dogs. I had always thought that the blood test had to be taken 1 month/30 days after the initial vacination, but the vet was saying 3 - 4 weeks which had me slightly confused. Have had a look on the DEFRA site and it doesnt state any set times only to follow manufactures guidelines on when to blood test. Has this changed and if so is it a recent change?

No it as always been the same wording, the optimum time is around 28 days, but can be done earlier that that if the owner wishes

Currently it needs to be a minimum of 21 days from innoculation.

http://www.defra.gov.uk/animalh/quarantine/pets/procedures/support-info/vac.htm