Part timer wrote:
With an electronic HF ballast, the power saving would be a bonus to
increasing lamp life, better starting and ditching the nasty starters.

Close to unity power factor too and no flicker, so yes, well worth doing.

I've seen ebay item 250670139376

Ah, I've bought from that guy: highly recommended.

Philips recommend using short cable
lengths to one end of the tubes. What is the reason for this?

That seems to be a fairly common restriction on HF ballasts. Philips'
data sheet for that ballast is at
http://www.lighting.philips.com/gl_e...spec-touch.pdf
It gives figures for maximum capacitance between the wiring to the tube
ends and to earth at one end (p.2) - so that must be it. See note 5 on
p.6 for practical interpretation.

something to do with 30kHz. How short is short?

See above. FWIW I've had no problems with lengths up to 200 mm or so.

Anything else I should watch out for apart from isolating the fitting,
double checking work, etc?

Make sure you get a good earth connection to the ballast's earth
terminal, as well as to the case.

A reel of RS 183-9225 is worth getting if you have several fittings to
re-wire. It's the pukka 85 deg. rated 'fluorescent fittings wire' and
works well in the push-fit connectors. Saves a lot of messing around
trying to re-use the old wiring.

--
Andy