The OEM S54 Solenoid Coil pack is known for failure due to the solder joints inside breaking as a result of the vibration from the engine, among other failures.

These rebuilt coil packs by Beisan feature a number of improvements.

Information and replacement procedure can be seen here: http://beisansystems.com/procedures/s54_vanos_solenoid_procedure.htm

Price is £250, £100 of which is refunded upon receipt of your original coil pack. Installation requires part BS024, available seperately.