Hi
My earlier offer is still open if you would like to take the trouble to download some photographs.
I say trouble, as downloading photographs is not quite the simple process you may be expecting. First you need to create a library of your photo's in 'images', once this is done you can then download or to be more precise in this instance 'upload pictures' to the discussion thread.
Given the nature of your problems, you possibly have more than one root cause - ground water alone will not cause dry rot and you must have certain conditions which is causing stagnant air to develop in the floor void (No ifs or buts, if you had plenty of ventilation you possibly would not have the dry rot problem, I'm saying possibly as at certain times of the year conditions may become ideal especially as you have so much ground water present - all you need is the right temperature and humidity and you have yourself a problem.
First double check your air bricks - they may be clean but has someone blocked up the ventilation holes? Get a stiff piece of wire (metal coathanger comes to mind) and check out the ventilation holes to make sure they have not become obstructed (cavity wall insulation comes to mind, the stuff does a wonderful job but can create more problems than it solves!) Just check out a sample number of holes there's no need to do every hole! If the vents are clear you should feel some draughts somewhere in the ground floor of the building, if not then you have a problem with air circulation.
? Is the surface of the block paving at least 150mm below the dpc level? Should have asked this last time round!
Dry rot is not about how long it takes to form, the problem is getting rid of it once you've got it! If its caught in the early stages then not too much of a problem to treat - in your case its a bit worse and you would hopefully have got a professional company in to carry out the necessary treatment - it does get a lot worse, so no need to overly worry.
There's not much point in moving the gulley, however, what you could do is check over the gulley to make sure the 'trap' is not blocked - get hold of some rubber gloves with long arms and stick your hand down as far as it will go and if the gulley is clear your arm should go down to about your elbow (or thereabouts) if it goes any less then you possibly have a blocked gulley, you should also check out the soil pipe which runs from the gulley, however, first things first, if you have had to clear the gulley throw a bucket of water down the gulley and if it drains away rapidly you dont have a blocked pipe and there is no need to take any further action. If you aren't familiar with a back inlet gulley go to a local DIY or Builders Merchant and ask to see one and get the person to explain how it works, it is pretty common for gulleys to block up from time to time and they should be cleaned out once every couple of years!
I would forget all about your other proposals as there is not going to be any real benefit. The only thing left of any real effect will be a french drain - this is topped of with pea beach/shingle which allows rainwater to drain away easily - if you wish to dress it up add some rocks and alpine plants, or plants that don't rely on a lot of water. I will put together a little specification for you - on a later thread - its taken a bit of a while to write this one and I am hoping that I haven't timed out!
Regards