it sounds like the washer unit is stuck on the seating.
Obviously turn the hot water supply off.
Something I tried worked. made a 1 ins [25mm] high plastercine resevoir around the tap 3mm below the centre piece then half filled it with WD40. leave for about 1hr allowing it to soak through between the body of the tap and the centre bit you're trying to remove.
First you need a good fit spanner for the nut on the tap body. and a pipe wrench [Pliers type] and a thickish cloth to protect the tap body. fit the spanner on the tap centre and place the cloth over/around the tap body and put the wrench on it so you will be pushing on the wrench in the opposite direction of the spanner this will counteract the force the spanner puts on the body of the tap and will stop the body turning while your pulling on the spanner to release the centre piece.
If it still does not move gently heat the tap centre this often works. If the washer unit it still stuck in the tap body then flip the shaft with a screw driver to dislodge it. if it's very tight then I'm afraid it's a new tap job.
Coat the washer carrier shaft and the tap centre thread with vasalene before replacing.
bob
January 2005