If you are using it just as a quick fault finding aid, then it doesn’t really matter, any clear hose will do. If you intend to leave it in, then just search for clear fuel hose. I can’t remember the exact specs of what I bought, but it’s around 10mm OD, diesel resistant, and bought from RS Components.
Edit:
I thought it was PVC, but wasn’t certain enough to say that. I have just googled clear diesel resistant hose, and this popped up.
https://www.thehosemaster.co.uk/clear-pvc-diesel-petrol-oil-fuel-hose/
