Basically you need a strip with sufficient length and width - and that's about it.
See Safety Sense Leaflet 12
http://www.caa.co.uk/docs/33/SRG_GAD_WEBSSL12.PDF
which refers to CAP 428
http://www.caa.co.uk/docs/33/CAP428.PDF
Planning issues. Remember to keep an agricultural device in your hangar I mean barn
Keeping a movement log is also a good idea, if you do go under the 28 day rule and someone says otherwise, you have a document to prove so. If you do go over the 28 rule for a number of years and no one notices, it can be aiui used to help a change of use.
Also speak to your neighbours, especially if they have horses, explaining the minimal impact e.g. straight in and out, not over their horses or houses, minimal circuits and you're welcome to come up some time.
Are there any obstacles at either threshold? Can you chop that tree down? Does it have a conservation order on it? Do they belong to you, or the neighbouring landowners?