A brake bleed won't hurt but you say driving the car it brakes fine, I'm 99% sure it'll be the master cylinder as you don't mention a spongy pedal. No leaks and a slow pedal drop under pressure indicate fluid passing the piston seal. How does it go back I hear you ask, well the piston seal is designed to seal against the barrel and the harder you press the pedal the greater the seals effort but when you release the pedal the fluid easily passes back to its correct side under the spring pressure within the barrel.
You are also correct in your assessment of proving you have servo assistance.
I'm really surprised that Ledgerwood's on picking up the problem failed to investigate it, after all a breaking issue could result in a catastrophic brake failure with who knows what consequences. Melvin Rutter or Brands Hatch Morgan will be able to sort you out with the correct replacement part. Most cars, well the cost is peanuts but yes its a Morgan so its about 5 times the price, probably getting on for £100 ish you could also give Caine at Wolf Performance a call as he is a font of knowledge with all things Morgan.
You should only need to replace the cylinder element itself rebuilding it with the reservoir/connections and servo. It's not a difficult job and is easily accessible in a Morgan.
