HMRC has an Enforcement and Compliance, Debt Management & Banking (DMB) agent dedicated line.
It was launched on 28 April 2014 and is intended for the sole use of agents with queries about debt management and banking.
HMRC says the aim is to provide agents with a priority service. The telephone number is: 0300 200 3887.
Opening times are 8am to 6pm Monday to Friday. Details are here