Went to BMW specialist who diagnosed problem with ABS unit. Computer diagnostics showed left rear sensor but changing this did not clear the fault.
Sent my unit to ecu-testing.com for repair and it came back today (cost £250
- put repaired unit back into car and now both ABS and ACS lights come on straightaway. The ABS unit also makes a load noise when checking the system when the car is started! I'm sure that it was not as loud before repair...
Please help... will contact repair company for advice tomorrow and give update.