I had a dpc course done in 2004, the plaster was replaced but mould has now started to come back through how do i make sure this is a fault in the dpc and not a fault with the re-plastering
The simple answer is that you need to contact the company who installed the damp proof course and ask them to undertake a re-inspection - presumably you hold a guarantee.
A qualified (CRSTor CRDS) surveyor should be able to diagnose the source of the problem and if the installation is shown to be deficient, then you should be able to make a claim under the guarantee terms.
As you state, it could be the fault of the replastering and in the experience of this company this is the most frequent cause of continuing wall dampness. If a visual examination is insufficient, samples of the plaster used can be taken for definitive analysis of its composition to determine whether or not it has the required salt retardant qualities.