Oh Boy, I am not sure what to think. I will need to take all the comments and see what the general theme is. My water tanks
are not leaking, although I have had to have the feeder tube replaced as it was so badly pitted it was sucking air when the water level dropped. Plus we fitted a seagull IV filter system and the filter got clogged up with slime, stopping it from working. I will not sacrifice another filter until I can get rid of the slime as a minimum. I will also try to steam clean the tank with an extension pipe to hopefully reach into the baffled areas. Then I guess hydrogen peroxide as a final treatment and see where that leaves us.
My thanks to everyone who has commented. If I hit upon a good solution without having to replace the tank or cut the top off it to line it with something I will let you know.