Crikey!
Never experienced that before.
From the AIP:
9.4 Charges are payable for the attendance of HMRC staff outside certain authorised hours for any duties other than the acceptance
of inward report of the aircraft, and the clearance of passengers and their baggage. Details of such charges are published in Customs
and Excise Notice No. 112 (January 2004) or they may be obtained from the HMRC National Advice Service (NAS). Tel: 0845-010 9000.
A charge may also be levied if attendance by Immigration staff is required; contact the local Immigration Office for further information.
(My bolding)