Hi Graham,
Others may comment on potato related headaches, but all I would say is that it's not always about diabetes. Lots of things can cause headaches, particularly dehydration; I often find drinking plenty of water will resolve a headache for me.
From a blood sugar point of view, mash will raise your sugars more quickly than, say, boiled new potatoes so they might be a better choice.
I hope you are able to get a meter and strips soon, as that really is the best way to monitor yourself and work out how things affect you. Everyone's response to food is different, so some people tolerate porridge well, for example, and others don't.
In the meanwhile, keep an eye on things and if you get a headache every time you eat mash after trying it several times, well I'd give up mash.
The advice for newly diagnosed diabetics that gets posted here is pretty good. Keep asking the questions though!
Good luck.