Had this happen once to me too and coincidentally it was also when driving an MX5
I feel the North East has received a lot of bashing due to ross26, most of the roads around where I live are rural and unlit and I use high beams as much as possible.
What is it with MX5's and this! I was overtaking an old saab in my (now gone) 1996 1.6 that had been doing 40 in a 60 for a consistant 10 minutes I finally get to an area where i can overtake I pull out get alongside and he accelerates up as I'm passing, luckily I wasn't floundering with the overtake anyway so still got past. It did cause me to be pulling back over far much closer to the oncoming car than initially anticipated.
I probably should have eased off and got in behind once I noticed him accelerating, I was only 21 and a little unsure of the best thing to do at the time.