Update for those interested
Picked up a 2nd hand Garmin 18HD radome this week and fitted it Friday morning
Didn't have time to give it a proper test as I had a few jobs booked in, so left it until the Saturday
When the mfd's are switched on, none of them showed the radar option, checked power cable at radome end and it was dead
However, the heading problem I had after removing the knackered radome had been resolved, result !
I thought it was a couple of degrees out, so opted to carry out a factory reset at sea
Bad choice as all 3 plotters were then 180 degrees out and we appeared to be going backwards
Tried the recalibration procedure on the fluxgate compass a few times and got it back to 90 degrees out again
Got up on the roof and unplugged the network cable, left it a minute or so then reconnected and it was OK again
Which was a relief as we had to navigate some shallow water to get back in and it gets a bit confusing when your boat doesn't know it's arse from it's elbow
I couldn't trace the power cable as some panels need to come off, but will get it done as soon as I get chance
Also noticed something that never happened before, when at anchor and I switch the 2 older mfd's off and leave the newer one turned on, I get a message saying that the heading sensor has been lost ?
I'll get power to the radome first, then go from there
But, I may need the services of a teenager, they're usually pretty good with tech stuff