Hi @jamesgrainger120 yes that's normal for this stuff.  Seemed to do the job for me (for fin rot)

I didn't for the time I was using it, but if I did,  I would probably add the right dose for the water replaced. E.g. for the 20L or whatever you change (rather than the whole tank volume)

How long does this take to take effect? Will raising the temperature help?

Raising temp causes the lifecycle to complete faster.  Have you seen any reduction in spots at all? I'm not sure that's a good idea if you haven't, it might just make the infection worse.

I'm not sure how good this stuff really is for whitespot, i know it helps with velvet but not sure on ich.

If you can't get the fish out, you might want to look at plumbing a UV system in.  Doesn't get rid of ich but keeps numbers low enough that the fish can fight it off.