I bought a Dji p3s from amazon a few weeks back i have flown maybe 4 times i have noticed the last 2 times a message saying weak radio signal, I done all the calibration things & done a imu one too when i first got it. The first 2 times i flew it i didnt have any strange messages & it seemed to fly great i have only used it in beginner mode. Now when i turn it on it where
it says radio channel its flashing on & off red for about 1 min 30secs please help !

A replacement likely won't be any better.  P3S and 4K do not work well in built up areas with lots of wifi interference.   On my p34k it's rare to go more than 350 meters in the city without interference warnings.  

How about the environment you flew the drone? Since it's WiFi connection, it's easy to be interfered.
Please fly it in another open place, adjust the antenna during the flight, and fly it farther firstly then higher.

I live 20 miles from any city only ever flown on dartmoor where I live

It seems like you have another thread on this as well?

http://forum.dji.com/thread-91447-1-1.html

You really should stick to having only one thread per subject, as answers could either be lost or unnecessarily repeated.