If its lots of money, you may want to speak to a professional re the investment side.
For now, get a lever arch folder and poly pockets and file paper. Find all your past statements and sort them out (by year or by account or whatever works best for you) Then on a blank sheet, go through the file and write down every account etc, plus any accounts you dont have documents for.
Dont get bogged down with interest rates etc just yet, just make a note of everything you can think of. Then, when you get a minute, call up/ go online and see what rate the first one is, if you have the time, look up the second etc. Do give yourself a deadline to finish it though, maybe 1 week or something. Put the sheet at the front for easy reference.
Then, if you get an email or letter about the rate changing, write that on the sheet and file the letter/ printed email in the appropriate place in the folder.
I personally dont have investments, but do have savings accounts and try to make the money work as hard for me as possible.
What sort of investments do you have? (Im not qualified to advise which is best, but am studying it, and in fact, writing an assignment on this very topic at the minute)
I review my current/savings rates properly every 3 months but usually only change once a year or less. For simplicity, all my bills have been coming out of the same account for years - switching them can be a bother.
For best rates on savings and current accounts look at www.moneysavingexpert.com/banking. If you sign up to the newsletter, they have info on new best buys in there too.
As for ISAs, you can usually move all your old ISAs into the bank your current one is in, which will make it easier for you to keep track. If you dont have an ISA now, research now and open it. While its best to open it on the first day of the tax year and whack the full amount in asap, you can open it any time of the year - right up to the day before new tax year.