Might any of you knowledgeable members be able to tell me what the recommended size battery is for a 2004 (last of the rover engined) Plus 8? thanks
John, Expensive batteries and unhappy placements in the car don't help your mogging or reliability. (There is a cute Morgan story about your era's placement but I will keep this short as I can.)
In short I gave up buying exotica and I use the traditional battery placement at the axle. The stock placement (unless it has been changed) rightly bothered the late Colin Jones who used to post here under the name SpannerJuggler. The two of us were very close once, I have hangover memories to prove it.

Colin, with your car, designed a proper battery placement. Peter Mulberry agreed and sold it as a kit with Colin, but that era is long gone. I co'opted by them as I used something similar early that year. I posted their article on GoMoG. Which I will re-post now just as they sent it to me.
https://www.gomog.com/allmorgan/batterytrayinstallation.html It is really quite easy to have made..by anyone or yourself.
I use mine with hinges the tray swivels down and drops the battery on the ground. All the holes in your suspension hoop you need are already drilled. Take a look. This means you can buy any battery you want! I choose the easiest battery to find in the area I am mogging. That will inevitably save you precious Morgan times and LOTS of money. I prefer batteries with amperage over 900 cranking amps.
Manufacturers recommendsbetween 600 and 800 Cold Cranking Amps (CCA).. You can find any size you wish with the table's indicated type number.https://www.gomog.com/allmorgan/CarBatterySizes&BatteryCutoffSwitches.html#NUMBERS For example the Odyessy 925 Battery code number BCI) is a 56. Others will suit your car as well or better as Colin found out and I found out. About 50 quid is what you should be paying rather than 200 and you will have a better balanced car and less risk of a ruined day.
You can use the newly emptied space on the bulkhead for picnic sandwiches or tools, like I do.
Lorne