Hi Martyn
We've just given up our seasonal tourer pitch after six years due to flooding, so my first piece of advice to you is that if your pitch is close to a river keep your eye on the weather and be prepared to go there at short notice to move your caravan, whatever you do don't rely on the site warden to phone you.
You need to make sure that if your awning is going to be left up permanently that the awning poles are good and that the awning is put up correctly. If there is any sag in the roof at all the first heavy rain storm will at best stretch the roof panel so that a pool forms on top of the awning and at worst cause the awning to collapse completely, bending the support poles and possibly damaging the side of your caravan.That is the most common occurence on seasonal tourer pitches.
I wouldn't leave the ehu plugged in personally because if the site staff cut the grass ( as they should do) they might damage the ehu cable with the mower. I would disconnect the gas regulator from the bottle when you leave as well in case , heaven forbid, a fire breaks out in your own or a nearby caravan.
Have a good look at what you are getting in to before you hand over any money. If the site was waterlogged and unusable are you entitled to a refund? What sort of people are on the site?
Do you have children? Do they have children? Are there people on the site nearby who are likely to annoy you, or you them?
Are there things to do nearby, and if there are licenced premises in the vicinity how will this affect you?
There are all sorts of things to ask yourself before spending several hundred pounds or more.
It might be worth taking a strimmer with you to cut the grass round the bottom of your awning and under the sides of the caravan otherwise you are relying on site staff not to cause any damage. Have a good look round your caravan for any little dinks and scrapes that may already be on it, then you would know if any fresh ones appeared.
I don't want to make this sound like a litany of doom, but there are some pitfalls and it pays to be aware beforehand.
On the other side of the coin, in our case we have made some firm and lasting friendships on the seasonal pitch, which was like a small weekend community. We have gone out for Sunday lunches a dozen strong, and got to know the local traders, especially the pub landlords and their regulars, quite well.
The main thing is to enjoy!