I didn't realise that de-activation was to do with the license. I thought it might be just to disable the software temporarily.
Anyway after hours of trial and error I think I've located the problem but I don't have a full solution, just a workaround that would prevent having to restart FSX. To re-iterate the problem, it is that within approximately a 10 mile radius of Central London all the icons disappear as do the Couatl menu options. Pressing Ctrl+F11 becomes inactive so there is no way to get them back without re-starting FSX. Emerging from the 10 mile radius does not restore the icons or the menu options.
The density of information is not the problem because it only goes wrong in Central London as far as I can tell. There is no problem in Central New York for instance, nor does it make sense to Restart Couatl since at that stage it has not yet stopped.
No, I've identified that the problem lies specifically with the Facilities Icons, not the Geonames POIs. I'm not sure whether this a general problem or just specific to my set-up but for the benefit of other users who may be encountering the same issue, the workaround is to select "Hide Facilities icons" after pressing Ctrl+F11. This must be done BEFORE entering a 10 mile radius around London. You can switch them back on AFTER emerging out of the "danger zone". If you don't do it before entering the zone there is no way to get them back as far as I can see apart from restarting FSX which is a nuisance.
The problem is with the Facilities icons. If you hide the Geonames but leave the facilities active it does NOT solve the issue.
Hopefully, Virtuali will address the issue if it is brought to their attention.