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.
I'm glad you were able to import the private key. But you may consider waiting for a period with less network congestion, and consolidating all the inputs into a single input with a low fee transaction. Since you are in no hurry, you can wait days for confirmation if need be. That way, when you do want to spend them someday, you won't run into the same problem.