Last couple of times I have been down at the test track it would've been visible from the far end of Asfordby Business Park at the Asfordby end. That was a while ago now. If it's gone to OD proper, then I don't think it would be.
I recall barriers for the business park car park but you can get in, no fee for parking or anything. Just drive in. The sidings are at the far end of the business park, alongside the Serco shed, with the Serco car park betwixt the two.