Question:
Can anyone suggest which are the best water purification tablets, please ?

Answer: My thought is why bother unless you know that the tank has been contaminated. There is very little that will harm you that can grow in the dark of a water tank. After a lifetime of drinking from boat tanks including the last 14years living aboard I have not once been hit by a bug that can be attributed to "dirty water"

We only dosed our water tanks if the bacteria level rose above a certain level. Remember our water came from a standing main in Lowestoft, through a big flexible hose between the quay and the boat that was coupled up, dragged around, etc. Sloshed around in the tanks of a supply boat for a week, pumped up to the platform again through a flexible hose that was left hanging on the side of the platform,maybe dunked in the sea and maybe had something spilt on the end of it. On my boat the water goes from a tap, through a hose that has been stored inside and flushed before use, straight into the tank of my boat.