Hi - I know the maximum that you can put into an ISA is �3,600.
When I get to say, �3,500 in my ISA (of which imagine �100 is interest that has been paid so far) can I only pay in another �100 so the total including interest is �3,600 or can I pay in another �200 as �100 of the �3,500 that's in there is interest, i.e. I didn't pay it into the account, my bank did??
You can invest up to �3,600... don't worry about the interest, as you say the bank/building society add that (tax free), so you end up at the end of the year with �3,600 + interest... the following year you can add a further �3,600. If you withdraw any money though, you cannot add this back in if you have already invested your maximum annual investment of �3,600.