Ahhh, Derbyshire...a great corner of the world!
Personally I haven't tried the JD Jetting kit but I've heard alot of good reviews, a bit expensive for 2 needles and 4 main jets, but it should put you somewhere very close to the correct jetting. For your bike you'd need the JDG003 kit.
I've haven't seen them for sale in the UK, except on eBay with delivery from USA...
https://www.jdjetting.com/xcart5/jdg...category_id=65
On eBay... JD Jetting Jet Kit GasGas EC EX 250 300 2018 2019 Gas Gas JDG003...
https://ebay.us/M2p95p
Alternatively, if you don't want to spend that much or wait for delivery from the USA, you could try these two needles. I believe they are close to the Red and Blue needles contained in the JD kit.
Red = NECJ
Blue = NEDJ
Can anyone confirm if I've got that right?
The NECW that DocBrown has just suggested is one step richer than the NEDJ, so would also be a good starting point. See the chart below...
chart.jpg