It seems my 2007 Subaru Forester, with a 2.5-litre flat-four engine and for which I pay over £500 per year in road tax is fine, while my 1997 1.8-litre MX-5 (which I think has better emissions) isn't. Unsurprisingly my 1958 Austin-Healey isn't free either.
I wasn't aware of these rules, very interesting to know about them.