Being too cold in winter
A conservatory isn't really designed to be used all year round. This is why it's important that it's thermally separated from the house with external-quality doors and why you can’t extend your central heating into a conservatory that's exempt from the requirements of the building regulations.
Make sure that any walls and glazing (windows or glazed roof) are suitable to protect against the weather and are of a quality that complies with all the relevant standards.
Remember that the glazing in a conservatory is invariably a structural element and not just a glazed panel.
You also need to be sure that the glass is appropriately toughened in critical locations.
Overheating in summer
You may want to install blinds, shades or even a solid roof but please bear in mind that a solid roof may stop your proposed conservatory from being exempt.
If your roof is leaking or your door is jamming, contact the company you bought the conservatory or the door from in the first instance and if the outcome isn't what you expected, consult these "Which" advice pages: