I don't have any capacitors in my electrical gubbins box. Doing a little light reading, it seems that I could put a capacitor to charge from my 9v battery then discharge through my electro magnets.
Using my key switch, I have two positions. The first can be used to charge the capacitor and the second to release the charge through the magnets.
As I understand capacitors, they remember the voltage put into them and then build up to capacity with power. The 9v battery trickles power out at a low rate. The capacitor can dump it at a high rate. Thinking of it like a dripping tap, dripping into a bucket, the bucket will fill slowly. Emptying the bucket can be done faster. Indeed a bucket of water filled from a tap is my fail proof method of unlocking a blocked toilet!
So, the new idea is to keep on with the 9v battery but to add a capacitor to dump power into the magnets. I have no idea how much power I'll need so I'll have a look in Radio Shack for their biggest electrolytic capacitor. I gather a diode is recommended in order to protect the capacitor so I'll put one, even though the pp3 battery connector is unidirectional.
It could be that I'm barking up the wrong tree with those magnets. They could be junk but it's worth a shot. It's not an urgent task. Worst case scenario, I just say stuff it and wire the magnets via the bus 12v battery and see what happens. Given an adequate fuse, nothing should be that bad!
Meanwhile, I'm going to look for barrels tomorrow. Word has reached me via the grapevine that somebody locally has a pile of 15 gallon barrels for sale. That's tomorrow's task!