Dub greg,
By way of location I'd say 75% probs it was a chinook, and 25% Puma. Although having said that it may have been AAC. 50ft unlikely, 100ft probable at night.
Either way be glad you are so well looked after. The taliban and Al Q are Suffering more than you at the hands of the other forces who may or may not disembark one of her Majesty's finest.
If you enjoy the noise - great, rest assurred that HM Forces are training hard to implement the policies of the government you voted in.
If you don't enjoy the noise - thats too bad.
If you feel strongly that the noise impinges on your way of life - GO FORTH AND MULTIPLY.
ML