Thanks for your response.
I've checked the agreement which I sign at renewal, and it is addressed to my employer with my name as contact.
It relates to the "loan" of a demonstrator and shows a cost and mileage limit - very similar to a normal hire agreement (which is what I did prior to taking a vehicle under this scheme).
Does this make it more or less likely that this is the reason why my employer considers the car to be a Company car?
Also, the tax liability shown on the coding notice (the most recent of 5 I have received so far this year) shows unpaid tax for previous years.
I was unemployed prior to starting with my current employer - for a period of 3 months (between November 17th 2012 and February 6th 2013), and recall that HMRC advised that unemployment benefit was taxable once they had been made aware of my new employment, however if the amount of taxable benefit at that time (as I recall it was something like £500), added to the £1094 on the P11d for 2013/14, my liability should be significantly less than the £2500 they are currently claiming as unpaid tax.
My employer has yet to submit the P11d for period 2014/15, but it looks likely that it will be for around £1500 - making the amount of tax due in total since 2013, something like £600, but I suspect that this will -to use your phrase - muddy the waters even further.
I'll try the tack of asking my employer if the costs I am incurring for servicing the vehicle can be added to the total of payments I'm making as rentals, to see what he says.
After which, I'll try arguing with HMRC.
Its not that I'm unwilling to pay my taxes, just that they way in which this has been done leaves a sour taste!