If the fluid you need is JWS3309, you can buy 4.55L of compatible fluid for about £30 :
https://www.ebay.co.uk/itm/Carlube-A...L/233257297843
I would do 3 or 4 "Part changes" each a month or so apart. Drive the car for 5-8 miles to get the ATF properly warmed up.
Park it so it's level so you can both drain and check the fluid level. Arrange a catch can underneath (Silverline do some good ones for about a fiver posted, look on ebay), remove the filler/level plug and remove the drain plug - carefully as it will be hot, try not to drop it or let ATF run down your arm!
Allow to drain, clean the drain plug and refit once drained. Arrange a funnel and some hose to go into the level plug hole and pour fresh fluid in until it come out of the hole. Refit the level plug, take for a test drive and recheck the level, topping up if necessary.
I transfer the old fluid to a clear container (ex screenwash are good) and allow to settle. Once settled i check to make sure there is no coolant at the bottom of it. Pouring some of the old fluid in a dribble onto some folded white kitchen roll will reveal a lot about the condition, colour and amount of sediment in it.
Repeat for a total of 3-4 changes a month or apart then reset all the fault codes and check the fluid has remained clean this time. Dipping your finger in the level hole and wiping on white kitchen roll will show you.
Repeat at 12 month/12k mile intervals to keep it fresh. It's a lot cheaper trying this than throwing it in for a rebuild and should save the box if it's possible to save it.