My toilet is continuously filling but I can't find the float. There's the tube with the buttons, an overflow tube and a sealed grey box connected to a tube to the bottom of the cistern inside it. The grey box has CL and a line on it, so I presume it gets the level from there. But it doesn't unscrew or unclip easily so it's hard to tell. How can I adjust the fill level please?

I assume your toilet has some sort of Fill valve assembly. On the top of the valve there should be a screw that you can turn clockwise or counter clockwise to increase or drop the water fill line. Any way to snap a pic?

Pic would help one searched online would be best. Fill valves go after a few years and are very easily replaced. In US or elsewhere? Local folks would be of help if the assembly is different standard.

Some valves have a box at top with a tiny float inside. I prefer the cylindrical float. just adjusted the kiddo's the other day, it was filling whenever it felt like it.

If there is no ball cock than it is either:
A float cup toilet fill valve
A floatless, pressure-activated and anti-siphon valve
or
A pressure- and vacuum-assisted valve
If you do not know about these, it's probably best to just replace the entire toilet

Where's the toilet and what are the local/national standards?

Here in the US, there are almost universal replacement part kits available for under 30 bucks that restore a toilet to brand new condition other than the ceramics. I like the kit that includes a flush lever that fits the vast majority of tanks, that one part in the kit alone costs a minimum of about 6 bucks.