The running costs of a shed in Engkand would be huge to heat unless you used more suitable insulation, celotex is the product to use, 2 inches is like 4 of the fluffy fibre glass and is coated in silver foiled to reflect heat. Expensive but a one off cost as opposed to ongoing massive heating bill. 5 300 watt heater wouldn't keep up in winter. You also need to cover the pond and use a double glazed window and the very least a curtain before you close the door.
The pond would have needed to have been covered for an aro too....sorry to hear that it died.
At least this project is a good lesson for us in terms of what is needed.
Thanks for the updates