Afternoon,
One of the drives on my FS was showing as red yesterday. I ejected the drive, put it back in and then there was no light at all.
I rebooted the unit and it immediately went into rebuild mode (I think). I say I think as it cannot be seen by the dashboard.
Do I just wait for it to finish? It has been going for 24 hours and was at around eighty percent capacity.
If you suspect a drive has failed, and your Drobo is not happy, remove the drive and see if your Drobo recovers.
Rebuilding is fine, but it’s a bit worrying that Dashboard can’t see the Drobo.
P.S. I love your nick. Bluebird! 
Was it blinking red, or just solid red?
It was solid red on drive 3.
I removed that and put it back. The red then moved to drive 1.
I turned it off and then it started blinking all 5 as if it is rebuilding.
Solid red just means “add a drive here or replace with a larger drive” - it means Drobo is running low on available protected storage.
http://support.drobo.com/app/answers/detail/a_id/158/~/what-do-the-led-lights-next-to-each-drive-bay-on-drobo-indicate%3F
There’s (probably) nothing wrong with the drive, but you should definitely consider upgrading drive capacity or offloading some data.
Nothing appeared to be happening so I powered down and back up.
Came on with the same solid red on drive one but then started rebuilding across all 5.
It is now seen in the dashboard and quotes 72 hours for a rebuild. However my computer and all network devices cannot see it.
Should I just wait for the rebuild to complete?
Yes, wait for the rebuild to complete - actually there’s not much else you can do. 
Older Drobo units, especially FS and the 4-bay units, can be really slow to respond while rebuild is in progress.