Also, there's nothing magic about the MOSFET, all you really need is
a logic level N channel device capable of handling the solenoid
current when it's turned on and holding off the solenoid voltage
when it's turned off.

Hi John, I would like to get back to the MOSFET comment. This sounds
scary to me. About the current. Do you mean, as we both calucated that
if the 72V will use 3A current, the then mosfet may need 5V 3A, in and
that will allow the 72V 3A out?

So my pic needs to give 3A??? This sounds almost impossible. I'll have
to check the datasheets. Is this how all transistors work re current:
The current in the base needs to be the same as the current you want
in/out on the collector and emitter?

And then of course, my next question will be if you can suggest an
appropriate mosfet from my local shop: www.radionics.ie

Thanks & Regards
Evert