Thanks for your advice, Moonbeam. Last weekend I looked more closely into the problem. The anode is connected only to the saildrive – no protection for the keel.
I talked to another who owner had cause some years ago to seek advice from MG Duff, who said that the area of a keel is too great to effectively protect by anode, and the only real solution was to physically protect with a good primer etc. I shall do this at lift-out but in the short term I will set up an external anode and connect it to the keel bolts each time I return to my berth.