Paul,
There is a way to import TWC markup or wiki text, have a look on the
tiddlywiki.com for "classic" however the macro calls etc.. with not work. Hopefully you previouse wiki kept the data separate from the logic. However this is not as daunting as you would think because much of that which needed macros in TWC is built in to TW5
I put off making this move for sometime however TW5 is now so comprehensive this is in my view no reason today to delay, however it will feel difficult for a while as you readapt to the new concepts, however you will come to be able to do much more than ever before and more quickly.
I personally recommend building prototype equivalent things from your TWC in TW5, then once you are confident bring them together and only then organise to import all you content from the TWC.
Do start a thread as needed for help, but perhaps first make a local copy of
tiddlywiki.com and read it, experiment and play.
Regards
Tony