Hi. Pipe diameter is decided by the type of burner you are using, the hight of the storage tank (the head) and the length of run. in most cases it turns out to be 10mm annealed copper pipe and you must use flared type manipulated joints. As you are putting it under ground it will need to be the plastic coated type meeting standard BS5410 and avoid using joints under ground but if you do they must be accessible. The pipe must be run in a continuous rise towards the direction of flow and if it passes through walls must be sleeved and sleeved end sealed with silicon. A oil filter/water trap should be installed they must be made of metal and comply with oftec standard OFS E104. A remote sensing fire valve must be installed as the fuseable head ones are no longer acceptable when used on there own. but i alway take a belt and braces approach to these things and a fuseable head valve is fairly cheap. Depth of pipe is minimum of 750mm to stop freezing but i have seen as little as 100mm which is not advisable. hope this helps! Buster