I'm now El Capitan. The mountain did truly come to Mohammed. Took about six hours to download. Installed fine and everything apparently peachy on my desktop. Less stupid font. For some reason it bought back some desktop backgrounds that I used about a year ago.
For some reason the Macbook failed to connect to iCloud on installation which meant the email settings were inactive. Unfortunately they then refused to believe my password was my password. Had to nuke and re-create. To be fair, I think that's BT's fault, it used to be Yahoo Mail and now it's not, but my Macs have trouble forgetting the old Yahoo details.
Anyhow, fairly painless. Not much to report, everything seems to work as before. New improved font, everything feels very spritely but neither computer was very slow to start with, and various bits and pieces. Incremental and no Windows 10 tale of woe. Have yet to check if it fixed the bluetooth audio that Yosemite so handsomely and thoroughly broke.
PS if you have multiple Macs and don't fancy multiple 6 GB downloads, grab a copy the installer file from applications before you install, it can then be copied to subsequent machines. Once you install, it deletes the installer.