There was a time when we would cut the end off and put a new end on it.
That is a solution but IMHO its temporary at best because the simplest terminals tend to loosen.
A better solution is to crimp on a large ring terminal and use a battery connector designed for ring terminals.
The attached video has very solid techniques for making battery cables or in your case replacing the end.
Replacing the cable is difficult for the non experienced, and since you are asking.....

 

A better solution is to crimp on a large ring terminal and use a battery connector designed for ring terminals.

I've been using these for replacements. I went HAM though, and as I am OCD on electrical, I do have a rather expensive hydraulic lug crimper that I used to make a ring terminal lug for use with that.

http://www.fastronixsolutions.com/Battery%20Power%20Accessories/military%20battery%20connectors%20BPA.htm

 

What gauge cable should I look for?

You should be good with 6 AWG pure copper welding wire, which is more or less the standard size I've seen. Do NOT buy "copper" wire from Scamazon or Fleabay. Filled with crappy China made CCA (copper-clad aluminum) wire, which has much higher resistance than pure copper.

Temco is a great brand, made in USA.
I think I've bought over 200ft of wires from them so far, for solar RV wiring.

TEMCo 6 Gauge AWG 5 Ft Black + 5 Ft Red Flexible Pure Copper Made in USA Welding Cable Battery Wire - Heavy Duty Solar, Inverter, Car, RV, Truck

temcoindustrial.com

When you make a butt-splice, for the positive wire specifically, you want to be extremely careful and put as much insulation as possible. I'd use marine grade heat shrink tubing and put 3 layers.

Keep the spliced length of wire as close to original length as possible. You don't want to increase voltage drop unnecessarily from longer cables.

 

Really appreciate your advice here.

I'll take note on purchasing from a reputable supplier - also great idea on doubling up the heatshrinking and using marine grade!

 

Here's a picture of the terminal, you can see the corroded wires just underneath, wondered if I could chop back and put a new terminal on instead? Would mean stretching the wire I guess, the loom is really fixed.

 

Asking for further advice, can I cut the cable back say 5cm then crimp say the wire with a new terminal? I assume that is not advised with the amount of amps running through this cable?

 

If you have virtually no wire slack left, that's pretty much your only option.

When I did it, because I also had severe corrosion on positive wire, I stripped the wire back 2 inches (at most), and crimped that to a copper lug.

Because I used a Fastronix battery post, which is longer than factory, I had just a little bit more slack left thankfully.

For your particular situation, here's another alternative I can think of...

Here's the site I used recently to get a whole bulk assortment of reducers for lugs.

Buy Splicer - Reducer Wire Lugs at LugsDirect.com

Buy splicer reducer lugs. Allows 14 AWG to 350 kcmil (4/0 AWG) copper or aluminum wire, and are CSA certified and UL recognized. Aluminum with tin plating. Made in the USA from domestic and imported parts

lugsdirect.com

For battery wire, this will work up to 4AWG (so you have the option of upsizing if you want):

IHI - P4 4 AWG Aluminum Splicer-Reducer

Buy 4 AWG splicer reducer lug with slotted screw. Allows 4 AWG to 14 AWG copper or aluminum wire, and is UL recognized. Aluminum with tin plating. IHI Connectors brand. Made in the USA from domestic and imported parts

lugsdirect.com

If you go that route, you won't have to solder or crimp.
You must insulate those VERY WELL, as mentioned earlier. A nick in the positive side wire touching the chassis (shorting the battery, basically) will result in huge sparks and/or fire.

Highly recommended: use Noalox to prevent long term corrosion:

IDEAL Noalox Anti-Oxidant Compound, 4 oz Bottle 30-026 - The Home Depot

Improve the efficiency of aluminum conductor connections by using this NOALOX Anti-Oxidant Compound from Ideal. Improves the conductivity.

www.homedepot.com

 

Another, safer option: Polaris Lugs:

Polaris 4-14 AWG Bagged Insulated Tap Connector, Black IT-4B - The Home Depot

Ensure a permanent connection in your boat with Polaris 4/0-Bagged Insulated Tap Connector Black

www.homedepot.com