As far as casting your parts, follow this link and read about some of the mold materials:http://www.smooth-on.com/
They have videos & how-to's that should get you up to speed pretty quickly. I am sure there are similar products available in the UK. Alumilite is a urethane I have used in the past, and it is good stuff. Check out their web page here:http://www.alumilite.com/
The only real tool you might need that you won't likely have laying around your shop is a vacuum pump and a vacuum chamber for degassing the mold material. For relatively small bits like you will be making, you can build a vacuum chamber fairly simply out of a "glass cake cover", a piece of flat metal (I have never tried it, but I have thought a plastic cutting board might also make a good base), a hose barb and some of the silicone mold material (to make a gasket). You simply drill a hole in the metal and tap it for the hose barb; then you mix some silicone and pour a "ring" the size of the cake cover. Let the silicone cure and you have a "vacuum chamber". It is best to use the vacuum chamber to degas a second batch of silicone and make a new seal that does not have "air pockets". From there you are ready to degas your silicone for making high quality molds. A vacuum pump can be purchased or built. I built mine using a "piston type" AC compressor from a car and a 1hp motor.
As far as tolerances go, the finished part will be EXACTLY like the original. Every tool mark, scratch, blemish, flaw etc will be captured precisely. The alumilite I have used has ~2min working time and parts can be cycled about every 5min. It will take a day, maybe two to make your mold(s). After that, you can make parts very quickly. With 5 molds you could likely turn out 60 pieces an hour w/o much effort.
I would suggest when you make your molds that you cast the bearing into the part. To do this you simply make the mold from a part with the bearing already installed. When making the part, you simply install the bearing into the mold as an "insert" and then cast the part. When the part comes out, the bearing is already mounted. You can actually even "cast" screw threads, but it is better to use a nut as an insert or simply use a smaller diameter smooth shaft and then tap the finished part (the screw threads in the mold do not last very many part cycles before a new mold has to be made).
Using "dowel pins" and structural pieces to support the mold material can significantly improve mold life and accuracy. For instance using a wood or metal base with a dowel pin the same diameter as your bearing ID as the "center" for your mold ensures alignment and fit. Same for your bolt holes, using dowel pins pressed into a wood or metal base will increase mold accuracy and durability. But for 1/4in thick parts, I would think a single dowel pin for the bearing would be sufficient support for the entire mold, or, if the bearing is longer than the part is wide, you could simply have the bearing "fit into" a hole in either the silicone or through the silicone into a wood/metal backing plate.
Anyway, if you really want to go into any sort of production on these parts, casting them is by far the best answer. If you will CAD the part up and send it to me I will make a copy or two on my CNC router (or on my manual mill, lol) then if you feel overwhelmed by the mold making process, I can make you some molds and send them to you, lol. Once you have the molds you could train a monkey to cast the parts. (Hehe, I might even try cutting the mold directly out of plastic on my CNC router; that might be fun;-) )
BTW, UPS updated the delivery of my steppers and controller to Today :-) ; so I might actually get the bulk of the assembly done this weekend! I need one more coat of paint....