On Thu, 28 Jul 2011 15:31:28 +0100, Frank wrote:

The older, lower ones seem to have got mottling or in some cases have
gone brown and died. Is this normal, or is that a symptom of virus?

If its a summer fruiting varieties, ie , the plants have given you a
harvest already, then its normal.
Remove last years canes to allow next years to grow.