I’ve managed to totally knacker the thread on my crank due to a poorly fitted pedal and it’s looking like a new crankset is required. Will I also have to change the cassette and chain?
Welcome to Bicycles Stack Exchange. That sounds like a rough time. Sorry to hear that. This might be a convenient time to replace those pieces if they're at that point in their wear life, but if they're in good condition there's no reason to.
Unior makes a pedal thread repair kit. If you're comfortable with thread tapping you could give that a look. Also, if you do replace your crank, you may be able to replace only the crank arms, and keep the rings.
Edit: Here's a link to the manufacturer's page for the item I mentioned.
A worn chain on new chainrings can lead to “chain suck” i.e. the chain failing to detach from the chainring and getting stuck in the front derailleur. A new chain on a badly worn cassette can lead to bad shifting.
Unless the chainrings are already badly worn I’d keep using them (if possible) and the chain and cassette. With new chainrings you might have to replace the chain (if it’s somewhat worn) and also the cassette (if it’s badly worn and won’t shift properly with the new chain).