I have installed the replacement PCB and it works great so thanks guys for the prompt replacement. I will send the old part back this week.
I have another issues now which is stopping me progressing further. When I try to home the laser ($h) as soon as the laser reaches the end of the gantry there is an error which stops it progressing (see video in link). Initially I thought that the belt was too loose so I have now tightened this up but the problem still persists. I have also unlocked the laser ($x) and then entered a few commands via the machine control tab; when I instruct 50mm movements in either the x or y direction it works fine but the problem persists when homing.
Any help would be great!
Video link: https://www.dropbox.com/s/af7t300dy53ftom/IMG_1860.MOV.mp4?dl=0