I was able to remove my BAHA "hat" (the large mastoid bandage) 24 hours post op. At that point, I could shampoo by holding a cup over the bandage that was held in place by the small, white, plastic cap. One week later, my doc removed the cap and the bandage and I was able to shampoo normally.
Once the bandage and cap were removed, he also told me to start putting ointment (bacitracin, neosporin, or polysporin) on the BAHA site.
Everyone's doc has different instructions - and Jo your doctor seems to do things very differently than doctors here in the States - so you might want to call him to see what he wants you to do.
The numbness will last a long time - the itching will drive you crazy, but only lasts a short time.