I'm embarrassed to say that many of the rooms in my house are lit by dozens of flush mounted halogen bulbs which must be very environmentally unsound.

Anyway, I recently "inherited" (totally bona fide, they were surplus to requirements) some low energy versions of the same thing.

My question is this - can I just plug them into the transformers that are there already, or do I need to buy new transformers (there were none supplied with the new units).

I'm afraid you will need to get new transformers. All explained here: http://www.energysavingtrust.org.uk/energy_saving_products/types_of_energy_saving_recommended_products/energy_saving_light_bulbs

PUtting in low energy light bulbs is a bit of a cost in the short term, but think of the long term gains...

Ah, well, we shall just have to bite the bullet. Thanks for the advice Fred.