I got caught out by this too. I knew my car tax was end of December but I bought my cage as an ex demonstrator so didn't know when it was orginally registered. I had been waiting for a MOT reminder last year & in November we were talking in office bout tax reminders etc and someone said that they don't send MOT reminders out any more..
checked on that link & sure enough cage should've been MOT'd in May!!!
quick call to local garage & got it done that day but i'd been driving without MOT for 6 months!! therefore no tax or insurance as invalid without MOT!! 22k miles .. phew i was lucky!! at least i won't forget again!!
as for tax I pay monthly on bike so as long as i have it MOT'd i'm covered ( and thats due day before my birthday so wont forget that lol)