Unlikely its wildlife, much more likely to be a persistent leak of some kind - I'd be excavating under there to see whats hidden beneath, unless your guttering's leaking and its been raining a lot (oh, if only...)

Sorry Bamboo, i didn't word this very well!!
We have had no rain, the guttering has been cleaned and checked and there are no pipes or drains any where near this section. It is not soaking or indeed saturated. The dampness soon evaporates in the mornings and stays bone dry all day! There is no smell either, it's a real mystery!!!!

If its a small leak in the water supply pipes, then it would be dampest in the mornings - no one's running any water and the pressure builds up, so more of a leak. When people are using the taps during the day, less pressure, no leak. That's what I think anyway - most people's water pipes run up the front garden somewhere...
I can't believe that you have an animal which pees there overnight during the night and has done so for 3 years and you've never seen it. And for it to rust through something, whatever's peeing would have to be the size of an elephant to have that effect.

I see your point Bamboo, but our water supply to the house is on the other side of our drive some 10 feet away. We don't have large water bills so i can't see it being a leak. Would the hydrangea not flourish if the soil was nice and gently damp with nothing more than water?

We have tall gates at the front of the drive, i don't always close them. I think i will experiment on closing them at night for a few days to see what happens. Could it be bitch urine?

If it was an animal peeing repeatedly on the same spot you'd be able to smell it.

I think there's some sort of water leak. I've found several leaks in water pipes through the sound they make - there's a faint hissing noise. Excavating - can be several feet - reveals a water pipe with a pinhole, or several holes. The leak gets much worse as you clear the earth off the pipe. Get someone with good hearing - young people have the best ears - and see if they can hear a high pitched sound - a very light "sssssssss".

If it's not your water supply, it could be to a neighbour's house.

Thanks Beattie, i'm open to any suggestions, we will try yours. I just don't see how a small leak of water kills any thing that is planted there. Even bluebells don't survive!

I think if it was an animal the smell would be the give away. Could one of your overflow pipes be dripping from the roof cistern? that would happen when you are all in bed and not turning on water.
If you are metered the water board will come and test for a leak. You turn everything off and if theres a leak the meter keeps going.

A continuous small leak can do an amazing amount of damage to plants. You can grow plants in soil, and you can grow plants in water (hydroponics) but few plants will grow in mud. The soil surface could easily be drying out during the day, if the leak is small enough, but it will still be saturated underneath. Whenever you have a "bad spot" in the garden, it's always a good idea to dig down to find the problem--it may turn out to be a large stone or layer of construction debris 50 cm down, stopping the drainage.

It could be a under ground spring or it could be a cracked waste water pipe if it's a old house it may have old stone/clay pipes

There also may be land drains in your garden/drive if it's a newish house (20/30year) from when it was a field???
water will still use the drains but when they build a house they cut through the land drains causeing problem later in time

I had a very slow leak under the concrete floor in my kitchen I knew nothing of it till the skirting boards in my dinning room swelled, horrible things slow leaks.

Thanks to every one! I shut the gates last night and guess what, we had a terrific thunderstorm and heavy rain for the first time in months!!!! Not complaining though!!!

I shall have to wait for a dry day and night again. Still can't see it being a leak, but will have a good inspection deep down. Will let you all know what happens.