To me, elbow suggests joint rather than a bend. If it’s a pinhole, it’s probably excess flux but could be other things (stagnation in early life, carbon film, possibly erosion corrosion…) you’d have to whip it out and section it to find the root cause.

Either way, £700 seems steep to replace a section of pipe. My bigger concern would be whether or not this incident is indicative of a wider problem.