I've seen the Themi whilst here in the USA on business,
and although I haven't used it myself, it gets quite
a good recomendation with the pilots who have flown
with it saying that they belive their rates of climb/thermal
centering have improved by using it. The only downside
I can see, is that the lights needs to be mounted in
such a manner that they can be easily seen from the
peripheral vision and not so low down on the cockpit
that you need to keep your eyes inside to see gain
the benefit, and in doing so compromise safety and
lookout.

They are about $1300 in the USA (about 750 pounds),
but don't forget that it is a GNSS flight recorder
primariliy (although not approved for world records
yet).

More info at: http://www.themi.de/

If you need to spend 750 pounds or so on a logger -
why not get one which tells you where the thermal centre
is!