I agree with Parabola, it is a beautiful gun and not only that the engraving is superb as is the entire filing and carving of the action. With such beautiful large scroll engraving that is done in the most excellent manner for the timeframe, plus the very fine top grade damascus pattern welded barrels, my guess is that the gun was a bespoke made gun that Leech & Sons ordered from one of the better Birmingham makers; and it was not made by Leech. However, since my libray of references list Leech being a gunmaker in Essex for a long period time, I think that by the time that this gun was made Leech was retailing guns made by others for his firm and not making guns. It is not the norm for provential firms such as Leech to have had the capabilities to make such an excellent firearm; there were some that could such as Horsley and Green. By 1913 fewer and fewer gun purchasers were ordering damascus barrels and this gun is special to my mind in that regard.
Bottom line is that you have found a jewel. Guns with long damascus barrels are in demand in today's market place, especially ones with good barrel wall thickness.
Please take more photos and send to us. Photos of the barrel flats, action face and the inside of the fore-arm will tell us a great deal. From the photos that you sent we can see that the gun was made with a Scott spindle and not the less expensive Greener top lever works.