[Fraggle]

OK so I now own. 0.53 of an acre of agricultural land in Devon, there is a hedge that abbuts the lane and is well established, the other three sides of the land are bordered by fresh air? As I need to establish borders, what am I allowed to use, both for short and for long term, I'm looking to plant a hedge, so what grows quick and will do the job, what can't I use? And what are my limitations,
Thanks for your time guys

[bj_cardiff]

What is the bodering land used for and who owns it? When you say bordered by fresh air, I'm imagining that its unfenced? So my priority would be a stock fence. I would look around and see what the other local hedges are made from and use the same plants. Beware that they will need protecting from livestock when small!

[Fraggle]

What is the bodering land used for and who owns it? When you say bordered by fresh air, I'm imagining that its unfenced? So my priority would be a stock fence. I would look around and see what the other local hedges are made from and use the same plants. Beware that they will need protecting from livestock when small!

The bordering land isn't used for anything as of yet,
Hawthorn, blackthawn, Holly,
Don't wanna poison any animals either?

[Fleecewife]

What is the bodering land used for and who owns it? When you say bordered by fresh air, I'm imagining that its unfenced? So my priority would be a stock fence. I would look around and see what the other local hedges are made from and use the same plants. Beware that they will need protecting from livestock when small!

The bordering land isn't used for anything as of yet,
Hawthorn, blackthawn, Holly,
Don't wanna poison any animals either?

Beware of blackthorn - it's vicious and spreads by suckers so your piece of land would soon be swamped.  The traditional wildlife hedge is made up of native plants mainly hawthorn, with others such as wild rose, an oak or two, beech, rowan, (not elder as it grows rapidly then dies leaving a large, difficult to fill gap, same with willow), wild apple and wild plum, hazel, field maple,  a holly or two, that sort of thing, but this doesn't grow all that quickly.  A quick hedge is willow, preferably woven for thickness and strength, but it takes a whole lot of maintenance and in its woven form is temporary. 
If you intend to keep livestock, or you wish to keep other people's livestock or wild animals such as rabbits or deer out, then you need to fence your land with a double row of stob and sheep mesh fencing, a minimum of 2 metres apart, with your hedge in the middle.  It doesn't leave much land for you to use

[bj_cardiff]

The bordering land isn't used for anything as of yet,

What is it likely to be used for? domestic, agricultural? The problem with hedging is that its very vunrable to being eaten until its very well developed, so you need to have a good relationship with your neighbours if they have livestock. I would want to put in a temporary (for 3yrs or so) electric fence a metre in from the hedge on both sides to give it a chance.

[SallyintNorth]

If it's unfenced now, I assume there are no livestock you need to keep out?   And on that tiny piece you don't intend to keep livestock yourself?  In which case you wouldn't necessarily need the stock fencing and can just plant hedging.  But if you think there could be livestock outwith your bit in future years then to protect your hedge you would want to put up stock fencing first then hedge within it.  And if you thought you ever might want livestock on your side of it - you could fatten 3 or 4 lambs each summer, for instance - then you might decide to put up the stock fencing on your side now. (It's much easier to do before the hedge grows, and you will lose more land to it if you do it later.)  Otherwise, you could plan to use strands of electric on your side for the few months each year you would have sheep on there.  It would be safe enough if you've got stock fencing on the outside of the hedge.

I would use Hawthorn as the main plant, with a few others as Fleecewife suggests interspersed.  And I totally echo the avoid blackthorn.  Its blossom is beautiful and sloe wine is my favourite, but it's a thug :/

[doganjo]

(not elder as it grows rapidly then dies leaving a large, difficult to fill gap, same with willow),

Juliet, there are elder trees round me that have been here longer than the houses - 50 years, so how long do they live?  Also I planted willow 10 years ago and it is showing no signs of dying so how long does it live?

[Fleecewife]

(not elder as it grows rapidly then dies leaving a large, difficult to fill gap, same with willow),

Juliet, there are elder trees round me that have been here longer than the houses - 50 years, so how long do they live?  Also I planted willow 10 years ago and it is showing no signs of dying so how long does it live?

Elder can grow huge - I was looking at one in our wildlife strip, planted about 20 years ago which is the biggest I've ever seen. It is though not in a hedge. The potential life of a hedgerow is forever. The way elder and willow grow, being more rapid than other typical hedge plants such as quickthorn, means that they shade out the slower growing plants over the years.  For laying the hedge, you then have no layable hedgeplants for a couple of meters on each side of the elder or willow, so there is necessarily a gap between the last layable plant, the trunk of the elder (which is not really an easily layable tree) and the next plant in the row.  As I've seen it, elders don't last as long as the life of the hedge, and farmers and hedgelayers will weed out any elder before it gets so big that it shades out the hedgeplants they want.  Willows can clearly last a very long time, as shown by the cricket bat pollards you can see along rivers, but they do the same thing as elder, stretching their gap in the hedge.  Once there is such a gap, even if you remove the offending willow, it is difficult to replant as the new saplings are then shaded out by the mature hedge around them - that is something we have tried with poor success.
Both willows and elder are fine trees, just not used as part of a hedgerow.

[edstrong]

No question about it - put a fence in, ideally a stock fence if you're considering livestock. You imply the boundary as ill-defined so a fence is the only way you will be able to identify the edge of your land and the beginning of your neighbours. Get in there now to avoid any future disagreements about where the boundary is.

Then and only then consider a hedge.