i suggest you speak to ACAS again before you take any further action
from what i recall, you sent your appeal by recorded delivery, and you had 7 days to send that in. did you receive a reply to this yet?
and with the data request, they have a few days to respond to this. did you send it by recorded delivery, and when? what is the deadline that they have to reply by? i wouldn't chase them up for this unless the deadline passes, and then there will be formal steps for you to take regarding the non compliance. chasing up informally may affect this, so stick to the proper procedures
what does the dismissal letter state as the official reason for dismissal?
btw, i didn't know you had other issues with your health. the employer can't send you home like that without pay. this sounds like further evidence of an unfair dismissal, the employer wants to get rid of you because of your health, which is illegal, and because of whilstleblowing, which is also illegal. you could have mentioned that in your appeal, but you can mention this in your tribunal complaint