While browsing on eBay I came across this device which is supposed to allow the use of 6ft tubes in 8ft fittings:

http://cgi.ebay.co.uk/6-8ft-Fluorescent-tube-extender-/160539793155?pt=UK_Light_Bulbs&hash=item2560ead703

Just interested in what people's thoughts are on this.
Apart from looking a bit strange, I would imagine that the 70w 6ft tubes would be overdriven by the 100/125 watt ballast unless the device in some way consumes ~30w to power the tube correctly. But then surely this would defeat its point of saving energy with its only benefit being that you don't have to replace the fitting when 8ft tubes are no longer available?

I'm pretty sure Kev bought one of these, I've wondered about getting one.

They're here: http://www.maxlumens.co.uk/product.html

There must be some sort of reactance in there, A resistor would have pretty high losses defeating the object really. You can't simply use a 6ft 70W tube on a 100/125W ballast, the tube would be overdriven and the ballast would get hotter than it's designed to.

The 6ft 70W tubes have a lower efficacy than the 8ft 100W ones anyway.

They look similar to the 'save it easy' T5 electronic adaptors.  Maybe they have HF ballast built in?

I think it work exactly like other such "energy saving" conversions:
Use tube rated for half wattage (and claim this in capitals), so give impression of half energy consumption, but causing higher losses in the original ballast, so the net electricity saving would be no more then 10%, but with shorter ballast life.

As it attach only to one end, they cannot use the "classic" electronic ballast trick normally used in T8->T5 adaptors (these tend to stress HPF RS ballasts due to operating them nearly OCV).
Very likely they contain only wires (so connect the tube directly), so overload both lamp and ballast (a bit).