magnet wrote:you can cast a concrete flywheel .......magnet

Abssolutely. Have you tried it? I do lots of concrete work. You need to build a
form. The form must be perfectly circular. So you have to bend wood to this shape. Perhaps a half-meter (or more) diameter. Plywood recommended. Regular wood requires steaming. Major issue, see other posts on steaming this forum. Even plywood might need steam, unless you can get your hands on 3mm plywood. And how shall we reinforce it? Wire mesh maybe. Or steel strapping tape, both of which I have used. If you build anything out of concrete you should have steel reinforcing because concrete is marvelous under compressive force but really bad at tension. That is why we try to reinforce it. That's why "rebar" which is short for "reinforcing bar" exists. And then you have to make sure the reinforcing is well distributed so's you have a balanced flywheel. I do not say that a concrete flywheel is impossible. In fact out there on the net there are plans for just such an animal. Google on "treadle lathe" and ye shall find it eventually. But it is a lot of work! Not as simple as adding water to DIY mix bags.
For the above reasons I think, but am willing to change my mind, that your best bet is to find a nice circular piece of steel or cast iron that you could use. I have in my shop an old automobille clutch plate. Just the thing. Alas it has an enormous hole in the center, at least 35mm and I need to plug it. But UK bodgers have a huge advantage over me. You have piles of Industrial revolution scrap lying around by the ton (or tonne if we metricate). Make use of it! It is what they call a "competitive advantage" in industry. Were I in the UK I would haunt scrapyards if I wanted a flywheel. Who knows. Your scrapyard hunts might even yield you a Brittania or Relmac treadle lathe intact! Most certainly will be rusty. Bearings almost certainly shot. We can deal with this. With much less trouble than pouring a concrete flywheel. Be grateful, Britons, and sally forth in searchg of flywheels.
Oops. Just thought of another flywheel source. An old-fashioned sewing machine. Built-in treadle and flywheel. Still some of them around; fast becoming collector's pieces in this country. If you find one you colud use it as the basis for a treadle lathe.