I think it depends on the DVLA with normal cars it's 3 years after first regitration.
I taxed mine last year for the second time without a new MOT, when I pointed out to the guy in post office that the car was registered September 2002.
If you've had a tax renewal form come through it should say on that what documents they require.
To be honest though unless you check your car over yourself on a regular basis as I do, it may be worth an MOT for your own peace of mind.
