Apologies for the delay getting back to you, and thanks again for looking at it 
I just got everything updated and did a quick test, and am still seeing the same response (i.e. customer not found) to the stripe web hooks.
Do you think I maybe should create a completely new pricing table / set of products and test with that, to rule out any issues that might be left over from previous tests (I did use a new user for this test, but the products are the ones that were there from before).