As previously mentioned, when out last Saturday and once since, I've had Bumble do a cough, where there is a momentary (and I mean momentary) loss of electrical power. The immobiliser light pops on and off, then all normal running is resumed. It happens in literally parts of one second.
I asked a reknowned expert this question:
Do you think this is an ignition relay glitch? I've had a look under the dash and there's a row of six relays with a big one (wiper delay?) on the left hand side. Or could it be an immobiliser glitch?
His (edited) response was this:
Firstly the relays on this genre of cars have not yet given me any problems, but wiring connections have. First port of call is the bunch of wires leading to the back of the scuttle area fuse box. These tend to be a bit short and can be only just connected. I would have the engine running and gently pull on these wires to see if you can reproduce the 'glitch'. May be worth removing these fuses individually to identify which ones give the cough result.
Next area of concern is the fuses themselves. Morgan rate all of the fuses a bit close to the maximum current draw for that circuit, this I have found to cause overheating of a fuse. Sometimes when a fuse is constantly heated and cooled it can develop a hairline crack across it creating a make and break situation. The other scenario is the sprung element of the fuse folder becomes 'softened' by the heat and no longer grips the fuse well resulting in a loss of contact now and then. All of this applies to both fuse boxes and not just the scuttle one. Inspect the fuses with a magnifying glass. Immobiliser systems are generally reliable except for the transponder falling out of the key, this pretty much stops the car though. I have experienced something similar on a plus 4, but this car cut out and took a few minutes to recover and start again. This was the cooling time for the fuse holder. Uprated the fuse a bit and all was well. One last thing, do not trust the listing for the fuses, check all...
So - first with the engine running I checked the bulkhead fuse box. The fuse which cuts the power when running is 30A second from the right. (Ignore the empty space).

I repeated this under the dash with the secondary board.
Two fuses cut the ignition power, both 15A and in this photo, upper row and second/third from the right.

I checked all the fuses with a torch and a waggle and all were intact. I then went behind the dash and with the engine running, did a lot of waggling and pulling, but could not replicate the cough. At this point I placed a neoprene buffer across the top of the steering column, because where the column swivels, there are two springs and the wiring loom is in contact with some hard edges. Finally I checked out the second main fuse. This is like the one next to the battery but 50A and shoved loosely in the corner. It wasn't pushed fully home, so I cleaned the contacts and fitted a cable tie in matching yellow.

I did another check on the relays. As a reminder they are Tyco Electronics 0-1432793-1, and are diode equipped. Furthermore terminal 87A (centre) is not used in this application.
I've sourced some spares from here:
http://uk.mouser.com/ProductDetail/TE-Connectivity/1432793-1/?qs=%2fha2pyFaduj8j%2fsWZguNLarOj0hnkzSrHDi%252bshaN3%2f9FOUBqDMX%2fgw%3d%3d
From the wiring colours I was hoping to establish which relay did what but it's not possible to isolate function. However number 3 from the left (ignoring the big one) is ignition, because the lights didn't come on with that one removed.
The big one by the way is a Wehrle 60 201 002 12v-5A, and includes terminals 53c (convention = washer pump), and 53e (convention is wipers) so that confirms what this one does.