Are you a higher rate tax payer, or are you likely to make Capital Gains that would leave you open to CGT? If you are neither of those, then to be quite honest investing in a Stocks/Shares ISA is not worth doing.
The last Government removed the ability to reclaim Advance Corporation Tax (ACT) on dividends paid on Stocks/Shares held in ISA wrappings. This effectively removed the tax exempt benefit of receiving dividend income from Stocks/Shares ISAs. Leaving only the exemption from CGT benefit.
In terms of where you hold your mini-cash ISAs, you should move them to optimise any returns, no matter where they are invested. I would advise against leaving any investment account dormant, move it and any new investment so your returns are optimised! Most ISA's that have been held for a while were attractive because they had an initial interest rate that included a bonus, which disappeared after a year, and now may pay 0.5% or less.
Some ISA providers give tiered rates based on how much you invest with them, just beware of the maximum amount that you can invest in any one Financial Institution before you exceed the FSA/Government protected limit, which I believe is currently £85,000.00
By the way I am also a wrinklie!