We take a heater every time we have ehu, not so much for the evenings but for on a morning ( im an early riser, about 5.30 usually) . I put it in easy reach so i can flick it on for 10 mins before i get up so its warm to sit and read with my cup of tea in comfort !
We had 10 days in our tent as Keswick 3 weeks ago and i am glad we took our fan heater. It is thermostatically controlled and as the weather wasn't the best, it took the chill off in the evenings. Definately take yours.