Looks like i need to sort out what i want to keep. There seems to be a lot of files on my phone I don't know what they are. Will it do harm to delete them, i think they're ones that have been somehow downloaded from the Internet. There isn't anyway of telling is there what your phone does and doesn't need is there?
No but there's the big easy button!
A lot of your don't care files are likely to be in a partition of your storage that may not affect your apps. Or they may.
Here's your strategy -
Helium Backup, factory data reset, restore your backup.
You just cleaned up any leftovers where your apps live.
On restore, that's the list of safe apps to remove - see any there you don't like - don't restore them.
Next comes your internal storage.
That's stuff that you can do a whole copy paste of the whole thing into a folder on your pc, just drag and drop.
I make a folder called "sd backups" and make a new folder in there each time. I name my folders TODAY'S DATE. Drag and drop all of your stuff there.
Now you can go through and delete stuff from your SD card and if you guessed wrong, you can drag the backup back.