All you can do is water and hope for the best. It may be too far gone.

give it enough water, spritz the leaves several times a day. If the leaves look like they would perk up if they were hydrated, lay the whole thing in a bath tub filled with water for several hours or even a whole day. I would like to also unpot and look at the roots. If they are brown and mushy, bare root it, cut off all dead roots and then do the soaking. If there are no or very little good roots, I would keep the stem in water till enough roots develop.

Keeping it damp may be not a good idea.

Whoa, that's a big one! I have a Dracanae lemon surprise but it's about a tenth of the size. Mine likes bright indirect light and to go completely dry between watering. Good luck with your stellar find. Hope he recovers!

I know! It has a price tag of £60 on it! as several of the flats in my block have been empty due to covid I wonder if maybe hes gone a couple of months without any care, we cut off maybe a third of his leaves which were all brown

Cool. If you get it breathing again, post a photo. Would love to see the before and after. :)

I have no idea what will happen, but if the dead-looking stalk has good live roots, I'd keep that one too. What do you have to lose?