Hi Nigel, congratulations and welcome!
Had the walnut dash on ours - lot of money for a bit of veneer, but you do notice it all the time, and for us, it just looks so 'right'. Divide the cost by the number of times/miles you plan to be sitting in the car, and I think that will help you decide.
Similarly, I was debating the cost between 4/4 and plus 4. The clincher in the end was being told that the extra money tends to stay in the vehicle - I think you'll find the same is true for the dash.
Why not ask the dealer and/or factory for a full breakdown of the differences between the runabout and 4/4? This must be information you are entitled to receive as a buyer.
Check out this link re the stats
http://www.morgan-motor.co.uk/sales/runabouts.html It may be wrong, but MPG looks a bit iffy when compaired to the figures currently given for the 4/4 - especially with less BHP. I assumed that they were using up old engines, but it could just be a typo, or something else.