When 3/4 inch is not 3/4 inch

When I ordered the replacement hose fitting for the calorifier I measured the thread on the fitting to be replaced. I naively assumed that 3/4 inch measured across the thread meant it was 3/4 BSP thread. When the part arrived I found the existing fitting will nearly fit inside the new one which has an outside diameter of nearly an inch. Turns out 3/4 inch BSP thread refers to the internal diameter of the steel tube it was designed to be on the outside of. My fitting is actually a 1/2 inch BSP thread. I'll have to visit the chandlery in person tomorow to try to find a matching part.
If they don't have one in stock I'll have to decide whether to start the delivery cruise tomorrow with the calorifier disconnected or leave it another week and risk not making it to Oxford in time for the start of August.