Why would you want to spend £100 ? You could make one fairly easily out of a steel wheel rim from a car. Split the rim in two and trim off the part that holds the tyre bead in place so the remainder of the rim will fit inside your spare wheel. Drill it around the edge so that you can bolt it to the bonnet. with adequate re-inforcing on the underside of the bonnet. Weld 3 long bolts to the cut down rim so that they line up with 3 stud holes in your spare wheel.
Cut a steel plate to go under the nuts that you will fit to hold the wheel onto the new mounting.
While you have the welder handy, weld a bar or steel loop on the mounting to take a padlock. Fit 4 suitable rubber pads on the bonnet, spaced out so they will support the spare wheel on the side of it's tyre.
Or......if you are lucky enough to source a scrap Series II or III deluxe dished bonnet complete with it's spare wheel mount, you could fit that mounting. I had one of these on my Series III.
Before you even start, have you driven a Land Rover with a bonnet mounted wheel ? If not, try placing your spare wheel on the bonnet now so that you can get an idea of what it's like to lift a wheel on and off: and also to see how much it might restrict your vision. And remember whenever you open the bonnet.... you will be lifting the spare wheel as well !
Hope this is helpful to you and good luck with it.