The OP has confirmed it was a Spitfire - EN830 to be exact - and not a 109.
We almost certainly saw copies of the same book. I'm still pretty confident about the author but after all this time I've no idea of the title.
I can guarantee it wasn't the Harleyford book about the 109. The Spitfire one was the only one I'd ever seen until three or four years ago when I picked up the one about the Lancaster in a charity shop a few minutes after it was donated.
Originally Posted by
El Bunto
Off a tangent from Merlins, but one early Bf 109 prototype was converted in 1938 with a P&W R1830.
Pprune won't let me link to an image, but search for "Bf 109 v21 radial"
Allegedly there was also a later conversion of a 109F with a BMW 801 radial but it's hard to separate fact from fiction there.
I'd never heard of it but that really doesn't look right.
Via one of the search links, it seems the one with the BMW 801 was the 109 X.