Got round to reinstalling this today ( nothing to do with the fact that I lost it having placed it on my desk - turned up last week stuck to a treasury tag!).
The hole in the float is open at the top, so I made a tool with a length of small bore copper pipe, the bore of which is slightly smaller than the magnet. Bored it about 5mm deep so the magnet was a loose fit. Put the magnet in the end and sucked on the orher end of the pipe to hold it in, then positioned it over the hole in the float and pushed it in with the pipe.
If I push the float down and turn the ignition on and start the car the dash warning light stays lit, but if I start the car and push the float down the warning light doesn't illuminate. Is there a time delay on the warning light or some other sequence involved?