James,
You can accomplish all the wiring you need for this installation under the dash. You need not string any wires back and forth to the battery or anywhere else under the bonnet. Neither the clock movement nor the lamp draws any great amount of current. Find any "always hot" wire under the dash for the clock. These are pure brown in virtually any British car. If you don't find one, use the glove box lamp power wire (I'm pretty sure this one's purple) Try to avoid splicing wires whenever possible. Join the wire where it terminates at the switch, or where it joins to a brown wire, by using a double terminal. Run a wire (brown would be a good color, or purple if using the glove box line) to the clock power terminal. Next, find the closest dash light and do the same thing, running to the lamp power lead on the clock.(I believe red w/ white is the correct color) Now all you need is a ground for each. Ground is always black. Use black wire, please. All this color matching stuff is to make trouble shooting easier in the future for yourself or whomever has to do the job. Wire both the ground terminals on the clock together (I'm surprised that it even has two), and then to any good ground under the dash. You may find a grounded instrument to share a ground with, or any screw that joins to the bodywork firmly. Use a true ring-tongue terminal and tighten it securely.
If you decide to put a fuse in the clock movement power line, use an in-line fuse and make it no more than 3 amps. The lamp needs no fuse of its own. Should you find that you absolutely have to join wires, use solder and heat shrink covering. This may all seem like a heck of a lot of bother for a clock, but it sounds as though you really like this car, and want it right. So do it this way. It'll be neater. It'll be easier to maintain and trouble shoot. And lastly, when you're finished you'll really like the result and will sleep soundly on it. Any questions?
You know, I've just reread your original post and I'm not too certain what is meant by "two spades for the light and two for the power". Are each pair truly separated from each other?, such that there are four separated leads? or do you actually have two pair of spades that are pairs joining the same connection? If so, the clock body may be intended to be grounded, like many British gauges, from one of the studs provided for the mounting brackets? Before we mess this up, gimme some more details.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon