Hi Again,
Well, all voltages appear normal: 12 at the lamp (Engine running, battery at 14v) No resistance in the chassis (-ve) line.
However, since I originally posted, I have discovered that my soldering to the pins was pretty rubbish!
Both the dip connection and the chassis/earth were, shall we say, "not my best"!
I remade them and now we'll see if the "bulb" lasts a bit longer for a change.
Again, many thanks for your help.