Give them a call, i've had a few that have taken ages to pay, its usually nothing sinister, just they don't see it as urgent, ring them and ask them and they will probably just say they forgot, then they will probably forget again so ring again and then send a final reminder through the post, if they still haven't paid by then then maybe they are trying it on. in that case get in touch with thomas higgins and they will send a polite but formal demand for the money, that should do it.