Google AI came up with this:

The brake discs for your 2014 Morgan Aero Plus 8 were originally sourced from the Mercedes-Benz parts bin, specifically from their high-performance AMG and V8 models of the early-to-mid 2000s.Like many low-volume British sports car manufacturers, Morgan heavily relied on major automotive manufacturers for standard braking hardware to pass global homologation rules.

Donor Car Specifications
The exact dimensions you provided directly match the front setup used across multiple Mercedes-Benz platforms:
Outer Diameter (OD): 345mm
Centre Bore (Centre Dia): 67mm
Thickness: 30mm
PCD (Pitch Circle Diameter): 5x112mm (which explains your measurement of "about 11.5cm" across the bolt circle)

Sourced Mercedes-Benz Models
When looking for direct replacements outside of Morgan's official parts network, you can look up front brake discs for the following models:
Mercedes-Benz CLK 55 AMG (C209 / A209) — Years 2002 to 2006
Mercedes-Benz C32 AMG (W203) — Years 2001 to 2007
Mercedes-Benz SLK 55 AMG (R171) — Years 2004 to 2011
Mercedes-Benz CLK 500 / C350 — High-specification V8 sport-pack models

Original Equipment (OE) Part Numbers
To ensure a precise fit, you can source aftermarket alternatives (such as Brembo, Bosch, or EBC) using the official Mercedes cross-reference numbers:
Mercedes OE Part Number: A2034210912 or 2034210912
Bosch Part Number: 0986479651


That's a good place to start i guess?

Current - 2013 Plus 8
Previous - 2002 Aero 8 S1, chassis number 96 - Ragusa Green Metallic