I'm happy to report that the replacement filter (AquaManta EFX 200) is working perfectly, and is absolutely silent. Good flow rate too! Thanks to using old media the tank is already cycled (pH 6.5, and levels remaining at 0ppm ammonia and 0ppm nitrite, 10ppm nitrate. The fish are happy as pigs in the proverbial now.
With fish in-situ and with ammonia building up, I'd personally be dosing the tank with Seachem Prime and Tetra SafeStart every 24 hours for a week or so. The former will instantly bind any existing ammonia and nitrite making it harmless to fish, while leaving it in a bio-available form for the filter bacteria to consume. The latter will help quickly seed the filter and 'flood' the tank with nitrifying bacteria of the correct species (not all products contain the right species) to eat what ammonia and nitrite there is. Win/win.
Jonny///M if you decide to do that, keep up the water changes and be aware it will provide false positive readings for a few days on the API test kit you're using, due to the method it uses to determine ammonia content. Regardless, the water will in fact be harmless to fish. Basically dose with Prime daily (as directed on the bottle) and add filter bacteria and wait. No need to test for that week because the Prime will have bound anything harmful anyway. I have cycled tanks from sky high ammonia and nitrite to 0/0 with climbing nitrate in about a week before now easily using this method.