Thinking of buying a patio heater, can anyone recommended a good one

Mage, I've had one for three years, I don't recommend the large gas type, they don't give out enough heat, OK if you are standing under it, but if you are sitting, a slight breeze blows the heat away, try a Chiminea, I think they are much better!

Have a north facing garden and can be quite coldat times looking to buy an outdoor heater cant decide on a chimenea or a patio heater? How long does the gas last in these patio heaters?

Both will put off a good amount of heat. If you want a hotter chiminea add more wood. Make sure you buy a single mouth opening chiminea for efficient drafting. They can really heat up and burn very clean. With a gas heater you just turn up the gas. The gas ones can range from a small personal heater to a large industrial. A typical patio gas heater will usually burn from 30,000 to 50,000 btu and a tank can last 10-15hours depending on the usage. You can get a burn time from the gas heater manufacturer since pretty much 1 BTU = so much gas. A chiminea is a great alternative if you like a nice fire centerpiece and want to make marshmallows or hotdogs with the kids. Our lake home faces north and the chiminea design protects the fire from the wind. It is made of cast aluminum and radiates heat on all sides but the highest heat is directly in front of it. Stay away from firepits, they are hard to keep lit, leave a ton of ash behind, and you will never use them in any wind at all. You will probably use a chiminea much more then a patio heater since it can also be a center piece for friends and family. The chiminea is also a much "greener" heat source.