Is a boot bottom required in the back? i.e. can the fuel tank and diff be exposed from the top? If not, can I just fit a pvc boot cover?

I don't know if one is needed, but a sheet of ABS fastened over the hole is enough to pass. My SVA inspector said that you could glue razor blades onto the boot lid and it would pass but he had to fail me on non-radiused reflectors. He seemed really sorry to have to do it.

Best to cover the hole really.

I passed SVA with just a cover on top

I passed with a cover alone. I did have to remove it so he could see inside though.

I covered my boot with a sheet of abs cut to shape, and secured it to the bodywork with industrial velcro.

I used 3 screws at the back and very strong velcro at the front - no problem with the sticktion (got it from B&Q).