Sigh.....

[tonystr]

will give it a go. What about a voltmeter? Is it necessary? If so, which way would I wire it up?

It's useful to have a voltmeter but not absolutely necessary.

Use a single pole double throw switch (SPDT). Like this

Black Slide Switch (16mm)

or


Slide Switch


Wire up switch as follows:
Connect centre tag to +ve of voltmeter.
Connect one of the end tags to 510 +ve
Connect other end tag to +ve of battery (after fuse to allow protection)
Connect -ve wires together. At the 510 -ve
Then with the switch in one position you can read battery volts, and atty volts in the other position.