1st layer issues

Hi, I've been determined to get the 1st layer printing correctly using a standard first layer test print. i've printed maybe 40 of them and made various changes throughout but the left hand side of the print is consistently more squashed than the right side of the print - consistently as in, every single print. I heat soak everytime, some of the tests have been done after a good few hours of heat as i've been doing test print after test print. If i raise the nozzle 0.01mm, the right hand side doesn't squash properly, if at all. if i leave the z offset how it is, the left hand side goes all blobby after a few lines as the pressure presumably builds up in the nozzle. I've decreased Z_probe speed to 2mm/s and have adaptive mesh levelling on. this appears to be working as if i wait in the height map menu, its empty until the printer has finished the mesh probing. I've tried printing the 1st layer test in different parts of the bed and it's always when the extruder is moving towards the front of the printer that it "over extrudes" and always when moving towards the back of the printer that it "under extrudes". Given that the PTFE line runs from the back i'd assume this to be reversed if the extruder were struggling - i've also tried pushing the filament into the ptfe tube while printing to see if it helps but its makes no difference. I'm out of ideas at this point, i've even rebuilt the hot end incase it wasn't sitting square with the bed. I've tried a variety of 1st layer heights and widths, all within normal kind of ranges that work perfectly on my ancient i3. I've also just opened a new reel of hopefully better quality filament and the issue persists. Any help would be greatly appreciated