My 80-claim patent suffered 40 separate restrictions under examination. I wish to add a linking claim in response, to overcome this. Can you help?

A linking claim could avoid continued restriction or perhaps consolidate a lot of your claims.  However, to link your claims, the linking claim might be too broad to distinguish patentably over the prior art.  You might want to conduct a pretty thorough preliminary prior art search that devotes a fair amount of attention to all the classes you mentioned.  If you do go ahead with a linking claim, take care to negotiate the potential hurdle of satisfying the written description requirement.  If your conclusion is that a satisfactory linking claim cannot be written, elect the group that seems to offer the greatest promise of patentability and greatest commercial potential.  In that case, you can pursue any non-elected group in a divisional patent application that must be filed during the pendency of your current patent application.