Colin notes that the drive is still a little off. Driving in a straight line is still quite hard to achieve. As hes been doing all the driving during the testing phase, I'll leave him to note down the specifics in a post soon after this, and hopefully someone can aid us in that area too.
The main issues I see are the castors and the wheels. The wheels are very large, close together and right at the rear of the robot. The robot is also quite long and thin. When large wheels are as close together as they are here, it's always going to be difficult to get the robot to go in a straight line; it's much more likely to have a tendency to veer off to one side (which side will come down to the dragging issue Colin mentioned) compared to a robot that has a wider wheelbase. Having a lot of the robot's weight towards the front of the machine and less over the wheels also means that it can possibly pull to one side depending on weight distribution etc.

The issue with the castors is that they reduce the rolling friction of the robot. In principle this seems good as the motors will technically have an easier job, but it does mean that when the powered wheels stop turning, especially when turning/spinning, the momentum built up will continue to rotate the robot, giving an oversteer effect. Most robots these days do away with castors and run with their leading edge just scraping along the floor. This dampens the driving and associated effects such as veering and oversteering. It means the motors will be drawing more current to move the robot, but I doubt the difference will be significant.

Obviously there's nothing to be done about the wheel width, it's pretty much set in place. You could try experimenting without castors if the robot's leading edge wouldn't hinder this (or try replacing castors with skid plates made of, say, Nylon) and see if that improves matters. The other option to consider would be the use of a gyro, which monitors the robot's movement along a certain axis and corrects any deviations. Never used a gyro before personally, but they can be a bit finicky to set up and use correctly, so you may not want to consider that option yet.

Either way, good to see the end is in sight. It's looking very grungy and agricultural, look forward to seeing it in an arena.