After quite some messing about I FINALLY got the Hipshot Ultralites fitted - with the KickAss also fitted (a relatively heavy bridge), the bass now weighs a total of 7-1/2 pounds. And more importantly, it balances really well on the strap. I tried to hide the screw holes made for the previous Schaller tuners, but you can still see the headstock has been Swiss-cheesed somewhat.
I filled the holes with Milliput (white) which stood out like white standy-out things, so when dry I coloured them in with an orange crayon, then a brown crayon, then used a pencil to try and incorporate them into the grain a bit more. Then I gave the whole stock a couple of light coats of dark yellow nitro. The result is more or less acceptable given the weight advantage (1/2lb), but ideally if you're going to use Hipshot Ultralites it would be a lot better to fit them to a virgin headstock that has had no previous tuners fitted. Or get a proper luthier to cut plugs and do an invisible repair.
The Ultralites are really good. These are the Licensed versions which can be got for £13 each. They're exactly the same as the USA-produced versions, but they're made in the far east. They do need bushes to make them fit a standard Fender-type headstock, but these can be got direct from the company.
Finally all done! Very pleased with this bass, it's light, resonant, the Entwistles sound really great, it looks good and it's currently my go-to instrument. Result. The trouble is, any other bass I pick up now seems ridiculously heavy.