Thinking laterally for a moment (it’s ok, I’ve had a couple of coffees so I won’t do any permanent damage)…
Without looking at the rules and regs of how you get these things done, is it worth looking at who does the speeding in the first place? Given how far you are from civilisation (and no, Malvern City doesn’t count), is it likely that the offenders are from a given pool of drivers that use the road regularly. IME it’s people that know a road well, but are only using it to transit from A to C and couldn’t care less about who’s impacted in B, are most likely to speed. I realise I’m in an urban environment, but you know my road. It’s pretty much the same few dozen cars that speed down it regularly. They’re using it to get to the long perpendicular road at the bottom which leads to a whole other chunk of Bris and aren’t bothered about children’s faces on my road. They know they need to crest that point where you can see the bottom or top so they then have right of way to proceed against any car turning into the road. It’s **** annoying to see the same cars do it. I’m tempted to buy a child sized mannequin and chuck it out in front of one of them one day. I’d just be worried that they’d skid and smash into a neighbour’s car. 🙂
So while some traffic calming measures might work in he short term, as the speeders get used to them, the average speed creeps back up again. Traffic calming might just be a new challenge for them! Out of interest, have you looked into how much it would cost to get one of those boxes that detect speeds and numbers (y’know, the things with the two wires across the road) so you can measure the problem? Surely you could squeeze a few quid out of the PC for one of those in the interim?