"Mike wrote in message

We've recently invested in a 10x14 greenhouse that has a cedar frame,
re-inforced glass & aluminium guttering etc. I have run a (protected)
240V supply to the greenhouse.

The north side is close (3ft) from a tall dense hedge, the East is
protected by a timber barn, but the South & West are exposed - we are
on a high hill in Kent. I have insulated with 1200mm wide bubble-wrap
(9mm cell size).

We have a load of tropical plants in the greenhouse - some fairly
hardy orchids, tamarinds, cactii etc.

Consider cutting your greenhouse in half with bubble wrap, using strips
hanging down in the middle as a door, effectively making two greenhouses.
The one furthest to the door can be heated to a high enough temperature for
the tropicals allowing air flow to keep the other normally unheated end
above freezing for the more hardy inmates like the cacti (provided they are
dry).
Seen that done with Orchids, hot end for Phaleanopsis etc, and the cool end
for Cymbidiums (which need cold but not freezing night temperatures to
initiate flower spikes).

A "normal" electric fan heater with internal thermostat would probably work
OK, it has for us for years, but if you want more control then a separate
greenhouse rod thermostat would enable that, they are widely available from
good garden supply co's.
Perhaps a second "backup" fan heater in the cool end set at "frost free"
might allow peace of mind.
Fans are always a good idea in a greenhouse anyway, just enough movement to
get the leaves slightly moving is great and will avoid many problems with
plant health associated with greenhouses.
--
Bob
http://www.pooleygreengrowers.org.uk/ about an allotment site
in Runnymede, fighting for its existence against bureaucracy.