Buy one of these and carry out the diagnostic interrogation yourself.
http://www.ebay.co.uk/itm/Any-Car-Engine-Fault-Diagnostic-Scanner-Code-Reader-OBD-OBD2-OBDII-CAN-BUS-D900-/350667477029?pt=UK_Diagnostic_Tools_Equipment&hash=item51a5691025
Admittedly it cannot do everything a main dealer’s laptop, loaded with the manufacturer’s software, can. But at £20 it should give a good indication of the engine fault.
The reason that the garages cannot detect the fault may be because not all faults are logged (stored) by the vehicle’s onboard diagnostics. If this is the case, it would suggest that the fault is minor in nature – and won’t cause serious damage, if not fixed.
Having the scanner to hand, will allow you to scan while the engine warning is illuminated.