£400 sounds high for a for a BM54. I would expect a BM54 to sell for around £350 and a used BM53 to sell for around £240.

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SOLD
2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input