Carlo's right about the "stress" illness, they're kinda stuck in a corner because disciplining/sacking them could make their illness worse, and lead to a claim against the company!
Someone at the last place i worked did it for ages.....!! Similar situation with depression.

As said above, speak to acas/cab and get their view on it, pretty sure they need to go through the disciplinary procedures 1st though.......verbal warning, written warning and stuff.

Cunts for even trying to sack you while you're physically unable to work!