Thanks for all the advice. I don't need access to the BTC - my motivation was to move it to a new wallet as Multibit Classic is no longer supported. Now that I've managed to import the private key into Electrum, I'm happy.

I intend to keep this BTC for as long as possible as I think Bitcoin will carry on increasing and if it goes up a couple more orders of magnitude it could make a big difference to my life.

Also, the guide I'd read about how to claim my BCH said I ought to move BTC to a different wallet first just to make sure - any advice there?