Thank you for your reply and the suggestions. However, you do not mention which "dirty contacts" you have in mind, or how I might check the "time the actuator is getting the pulse". I have checked the actuator function under direct vision many times, triggered by pushing the release catch on the outside of the tailgate. Subjectively, the tactile feeling I get when activating the catch is somewhat variable, but I am not sure whether this indicates any inconsistency in the internal function of the catch. I believe that it contains a microswitch, which is one place that there might be "dirty" or damaged contacts. You do not say whether the catch can be disassembled to examine the internal switch, and this sort of information would be most helpful. I am aware that if I take the car to my local Audi serviceman, the approach will simply be to replace "units", without any attempt to "repair" the problem. Perhaps this is the only practical way, but I am trying to save myself a substantial bill by attempting to analyse the situation in more detail. I would appreciate your further comments, and perhaps more specific instructions to complete the steps you mentioned.
Thanks for your assistance,
Thanks Billy, that gives me some very clear things to consider.
I'm pretty sure from my previous examination that the contacts on the outside of the actuator are fine. Someone posted on another forum that he had dismantled the actuator motor and found copper dust which, when removed, solved the fault. Although I inspected the individual components inside the actuator, I did not attempt to open the motor. I may consider doing that if it looks like my only remaining option is to buy a new actuator...nothing to lose! I will test some of your other ideas tomorrow.