Hi Graham and all,
I will use your report as a stepping stone to a suggestion:
Antenna problems are common in vhf installations and grow on the boat insidiously, often without anyone noticing in this day and age where cell phones are so ubiquitous: same with AIS. Most everyday vhf transmissions are under 5nm and are possible even with a corroded coax terminal. It is when you want to reach out 25-30nm and beyond (most likely not an everyday call but an emergency) that a little corrosion interferes.
At the beginning of the season, find a boat going in the opposite direction and set up a sked where you talk every 10 minutes or so and record distance apart as shown by AIS. Use the handhold also. Log where the signal gets scratchy and when it drops out for both the vhf and AIS. If there is cell phone contact, it may be that AIS lasts longer than vhf and you can continue to record AIS functioning. (If really energetic, play with different tx power levels to get a sense of the difference they make.)
Now there is a base line for the future and an indication of present functioning. If there is the thought that there might be a problem, repeat with another boat. Remember, the problem may have been the other boat’s problem.
Using the CG for radio checks is largely pointless and has to be a bother for them. It is pointless as their equipment for both rx and tx is so powerful that it would mask problems in your equipment unless they were really egregious: in which case, they would have been noticed already.
My best, Dick Stevenson