Working notice period

[Dixie68]

I'm currently working my notice period as I have got myself another (better) job. By my contract I have to give 4 weeks notice, but as my job is complex and I need to train my replacement to do it I have agreed to make it six weeks. I managed to injure myself at home last week (ankle) and as a result I have been off sick and won't be going back until this Wednesday - still within the self-certification period.
The thing is that I've just been told by a work colleague that my manager is going ballistic about me being "continually off sick" - I had a week off in July because I hurt my back at work (for which I had a doctor's note), and this period of 4 days. The manager has been saying that he will "take steps" when I go back in.
Now I'm not a skiver, I don't take time off when I get a cold etc like other people, and I've got to admit that this has pee'd me off big style as I'm doing them a favour by extending my notice period and training my replacement - I can't help the fact that I am off work because of a physical injury.
What "steps" can my manager legally take? I'm assuming he can't sack me as I haven't done anything wrong and anyway I need to train my replacement - and have extended my notice period to help them out. Also, if he starts on me am I within my rights to revert back to my 4 week notice period as that's what is written in my contract?

Many thanks