I can't see that crossover could give you such a large error.
With the figures you have given I assume the horizontal zero is at 20 yds.
This indicates that the scope and barrel are out of line by about 2cm at the muzzle.
Assuming your scope centre is about 44mm above the barrel you could get that error by canting the gun by about 23 degrees of the vertical or the equivalent combination of cant and scope not set truely vertical with the barrel.
.22 Webley Vulcan; .177 BSA MS Hornet; .177 FX Cyclone; 4 x .177 Gamo Delta's; .177 Brocock Hunter thumbhole stock; .177 Brocock Hunter Sporter.