Does anyone know a source of these? (Spearhead rather than concentric heat exchangers). Specifically I am looking for a pair 3/8" outside diameter (or better still 10mm to fit standard domestic plumbing fittings). They are or were made by Paul Gammon but I haven't seen one of his adverts recently.
If I can find a pair, I shall have a couple of unused 5/16" (24" long) to dispose of.