Looking very neat and tidy, looking forwards to seeing this at Grantham if you are coming.

Possibilities include:
Drilling holes in floor
Drilling holes in frames
Drilling holes in sides
Smaller battery
Lightening the wheels
Smaller battery
I would probably go with "drilling holes in the floor", "drilling holes in the wheels" (which I added :P) and and "drilling holes in the frame" would help. I wouldn't fit a smaller battery right at this moment unless you could match the capacity with a lighter battery.