Mashing temperature

[crh75]

How important is the mash/sparge temperature? I recently embarked on my first AG brew and last night I noticed that my thermometer was reading 10°C. I thought this can’t be right and checking it against another thermometer I realised it is reading about 12°C too low. Therefore I probably mashed nearer to 80°C rather than 67°C. The beer is fermenting fine and was at 1.014 last night (starting at 1.042). It had a rather bitter taste at the back of the mouth but I assumed this is normal and will go in the maturation process. Is this correct and not a symptom of mashing at too high a temperature?

I received a load of brewing equipment through freecycle and checked the hydrometer calibrations but never thought to check the thermometers!

Thanks for your time.

[crh75]

Thanks, very helpful. The thermometer may be more acurate at higher temperatures and so wasn't so far off as I fear! Anyway I'll be getting a new one before my next brew.
I wouldn't have said the taste was astringent so hopefully all be be OK in the end.