£800 for a plastic water tank!!!

The quote has come back for a plastic water tank. Only £808! Bit too expensive so I started looking myself. I found one from CAK tanks who make tanks with all fitments.. There largest is 423litres. That will do us and they cost just shy of £300 plus £21 delivery. A lot cheaper than £808!!! They take 3-4 weeks to be made. We are not in a hurry as we could fit it ourselves (along with the rest of the fit out). As long as the deck fittings are provided (filler cap and vent) I can't see a problem.