OK, just a bit of a rundown of what I have done in case anyone ends up in the same situation as me.
The main challenge is that Birdseye is designed to load aerial images to a compatible GPS unit, not just view them in Basecamp. So there are some hurdles if you, like me, use a good old 276c.
Firstly open Basecamp and there is a Birdseye menu with an option "Download Birdseye imagery...". Good so far, but Basecamp then searches for a compatible GPS attached to the computer. If you don't have one of them you need to fool it by loading the .XML file hosted here
(need to sign up to download it) into a folder named "Garmin" in the root of a USB key, so that it looks like a compatible Garmin unit. Then you can choose the Birdseye product (I used Sattelite Imagery Subscription V2).
Birdseye is a subscription service costing about $40 a year. I'd happliy pay that, but to purchase it online you MUST have a compatible GPS unit registered to you Garmin account. I don't, but you can use the demo version and a workaround. The demo version allows you to download only one aerial image (problematic, but can be fixed as described below), and will not allow you to transfer the images to a GPS (no issue for me).
Now to use the demo just click "Continue" and you can then select the area and quality you want to download. There are limits to the size of each download:
Standard quality: 5000 tiles, 57Mb, roughly 60km x 60km
High: 107Mb, 20 x 20km
Highest: 76Mb, 7km x 7km
Hit download and things get going. A single .JNX file will be downloaded, containing all the sattelite data selected. I find it takes about half an hour to download each 57Mb chunk of Standard data. Even underway you can see the data tiles pop up in Basecamp. And now you have some sattelite imagery over which you can plot routes and waypoints etc.
Now, using the demo version, if you try to download another area of sattelite imagery then Basecamp will warn you that the previous image will be deleted. This is only half true. You won't be able to see the imagery in Basecamp any more, but the actual .JNX data file will not be deleted until you quit Basecamp
So what you need to do is as you go, or at the very least before you quit Basecamp, is to copy all the .JNX files out to another location. By default the Birdseye .JNX files are downloaded to: %APPDATA%\Garmin\BaseCamp\JnxFiles
For me, using Windows 8.1 I found them in the "Roaming" directory. In my case this was: C:\Users\bc\AppData\Roaming\Garmin\BaseCamp\JnxFiles
If you want to store them elsewhere, on an external drive, or a simpler path you can set up a new database in Basecamp (Google how to do that).
So now you can build up a depot of .JNX files covering the area you are interested in. Though it is very labour intensive for large areas or high quality. To then view all the images in Basecamp again takes a trick too. I copied them to a "\Garmin\Birdseye" directory on my USB key that was made to look like a Garmin GPS. Then once this is recognised in Basecamp it will show that it contains all the Birdseye data you've downloaded. This can then be copied into your Basecamp library and used as you see fit. However be warned, if you download more Birdseye images you will no longer be able to see this data in Basecamp, it will need to be reloaded.
So that is what I've learnt from extensive Googling over the last couple of days. Hopefully it is helpful to someone else one day.
And Ken, I'll be riding a Honda XR650R. The last serious bike built