How necessary are mast extensions?
I am a newbie when it comes to windsurfing and decided to go out and buy a second hand board/sail/mast/boom
But do I need a mast extension?
The Mast is 430cm with the sail being a Gaastra sail 4.4 Radwave Luff - 450cm - 465cm. I wasn't given a mast extension but I feel like I do need one because isn't the mast meant to be the same as the LUFF ? Also it doesn't have a downhaul pulley on the sail.
Any help, much appreciated.

Yeah, you need one. That's the accessory that connects the sail to the board.

Not only is a mast extension required, but you will also require the fitting that attaches the mast extension to the board which includes the flexible universal joint. Suggest take a trip to a shop that sells windsurfing gear and make sure you understand what is required for a complete setup.

As for the sail not having a downhaul pulley....how old are these suckers? Back in the early days my sails only had a brass eyelet. To make downhauling easier I purchased a fitting that can only described as a hook with 3 pulleys. The hook part of the fitting hooked over the eyelet and below were the pulleys to connect with the mast foot. I assume you can still buy these?

Mast 430cm + extension = luff length, for example
430 + 20cm mast extension gives you your luff length of 450