Hi,

Bought my Mavic Pro 1 close on a year and a half ago.  Crashed it into a tree soon after (playing around in Sports mode) and put it completely out of action 'camera wise' until I had it repaired a month ago.  Then... one of the motors failed on my very first test flight.  On this time I took up the challenge, ordered the parts and repaired it myself.  Flight testing it today presented me with a problem that I'm sure the experts out there will help out by providing a simple answer that, at this stage, is beyond me.  
I have GPS ready to go, can set RTH but what happens is that I get a continualy flashing aletrnate re and yellow light.  It flys (or is the flies) normally I think.  My Manual does not give me a lead into what that means (or I've missed it).  

Can someon please tell me what's the problem and what I need to do to fix it.  Newby... seeking help.

Look on page 12 to 15 of the Mavic Manual.
I believe you may have to do a Compass Calibration (fast flashing red/yellow), as explained on said pages of the manual...

Calibrate the IMU and compass