"Paul" wrote
| I like the look of these new storage heaters - but before I take
| the plunge, any opinions on this as an option rather than extending
| the GCH system?

Go for GCH. Storage heaters in the extension will look cheap and be
inconvenient.

Microbore is limited in the heat it can carry. Your existing microbore rads
will be plumbed individually back to a distribution point, either a manifold
or 'ordinary' flow and return pipes from the boiler. You will need to run
pipe from your extension back to this manifold (hopefully there will be
spare taps on it) or the ordinary flow and return pipes, rather than just
teeing into a nearby radiator.

Owain