ACE - changing the upper accelerometer?

Since new (Disco II, TD5 GS Manual, 2002, now 90k miles) I've had my ACE warning light come on intermittantly to the yellow warning (apparently it goes red when theres a real problem) light - especially when on longer journeys on straight roads. It's been diagnosed as a problem with the upper accelerometer, which the workshop manual says sits at the top of the windscreen above the sunroof winder/courtesy light. After telling me where the component is, the manual then fails to tell you how to change the component!
Has anyone had any experience of changing this?
Is it accessible without removing all the headlining (i.e. just by removing the plastic surround/facia across the headlining?
Is this an easy job I could handly myself (reasonably competent diy-er with access to a decent toolkit)?
Help!

its above the front interior light, undo all the screws and drop the overhead console off, it sits in a bracket and can be prised out, dead easy, when you have fitted the new one, you'll need to have the accelerometers calibrated with t4 at a dealer