Although I was aware of the changes, most useful info was from the 2018 DT catalogue.
What seems to be evident is that sensible priced and smaller 'single seater' extinguishers won't be made any more. My 2.25 litre fitted (in FF2000) at the moment is listed (in 2017) at £231.45, and to get an FIA compliant bottle from the same manufacturer that will directly replace it now, the cost will be £425.00....... Another make does offer a bottle that could possibly fit, although longer and smaller dia, at £215.83. Seems to get a small system to fit a single seater you now have to have either a very expensive bottle, very expensive firing mech or very expensive extinguishant! Interestingly most extinguishers in the catalogue now suggest for 'GT / closed car / rally / saloon use', and it's difficult to spot anything for open cockpit / single seater use.....
One other point from the regulation changes is that bottles will have to be fitted to brackets with 'anti torpedo' tabs. (To stop the bottle sliding along or through the brackets.) If you have a DT or other catalogue, you will see what they look like. Think I could retro fit to an existing bracket, as long as that is allowed!