Not a Chim owner so may be missing something obvious but was this a barb style fitting for a vent or line? if so could it not be repaired? granted tank would need purged etc before welding but not a big drama.
Easy fix for a competent welder. Tank will need to be fully purged or full of water during the weld. If it is within your skill set you will bring the price down by doing all the prep work and just presenting the welder with a nice clean hole and a new spigot (ali tube B&Q).
