Help with Ransomes petrol mower?

I have a Ransome Sims and Jefferies PLC, model Marquis 51 petrol mower. Other wording on it - Briggs and Stratton 1/C, Industrial commercial engine, cast iron bore, 3 hp, quiet power.

It has started to cut out and will not start again. Has good spark and starts if I pour a little petrol into carburettor. Must not be drawing petrol from tank. Checked diaphram and looks ok. With diaphram cover off I blew down one porthole and petrol squirted out other hole so no blockage in tank.

Has anyone any ideas that might help me get it going again?