I would be very careful with guns from Simpsons. They find great bargains overseas and import economical guns for affordable prices. However, they often have guns that are off face, out of time ejectors, or dangerously thin walls.
A bargain basement, utility grade Birmingham double rifle with the meaningless Manton name attached to it isn't worth a fortune. A pristine example with ejectors, long stock, and plenty of case colors remaining might be $18k-$22k. If you misread a defect or problem with a $14k Manton, you might find yourself owning a very sad gun with $24k+ invested in it by the end of repairing latent defects.
Just be careful, make sure your desire for a deal doesn't cloud expert judgment required to buy such a gun that has weak documentation.