Rachel on top of the hydraulic pumps there are 3 Allen screws & a larger hexagonal headed one - that one is your filler plug. Do watch your fingers those visors are very heavy. It was a struggle for two of us to carry one.
Unlike a Land Rover with the hand brake operating on the transmission, the Humber has rods linking the handbrake to all the wheel cylinders. These rods are either seized, or the wheel cylinders are seized or the brakes shoes have got stuck onto the drum with rust.
If you are convinced the rods are not pulling the brakes on, try slackening the brake adjusters off & seeing if you can get a wheel to spin freely, then re-adjust.
You may encounter two types of adjuster. The earlier system with slots in two horizontally adjacent adjuster wheels accessed through a slot in the back plate, usually covered with an elliptical rubber bung.
Or you may have the later type which is a square nut head with a hexagonal one superimposed on it.
When re-adjusting do the lower slotted wheel first as it tends to pull the upper wheel. Similarly with the other type do the square nut first.
If you take a wheel off remember, the studs on the RHS are RH thread & on LHS are LH thread.