Wheelbuilding / Wheel Building / Wheel build help

Been having trouble with my rear tyre losing pressure overnight (gravelking+ on a hope 20Five rim). Thought I must have buggered up seating it but then spotted this bubbling (presumably a bit of sealant) from the inner surface of the actual rim itself. Looks like there's a tiny crack, not sure what from. Two questions:

• presumably the rim is buggered? I emailed the hope warranty people who said I could post it to them to have a look....not sure it's worth the time/hassle.
  
• can anybody recommend a north london wheelbuilder who would build me a new rear wheel on the hub (a 32 hole hope RS4, if that makes a difference)?
  

2 Attachments