From what you say, and given your house has pile foundations, I doubt very much it is subsidence, when water is removed from clay subsoil (usually by a tree) it shrinks or for that matter heave where there is excess water in the clay and it swells - due perhaps to a tree being felled or a drainage or water main leak.
Whilst it is true clayboard should have been installed under the reinforced concrete ground beams, I am of the opinion the damage such as it is could be caused by tree roots from the tree. This would explain the issues with your paving.
Certainly, what you have on your main house cannot be left and ignored and as you are outside Redrow Homes warranty two year period don't expect any help from them! You need to make a claim under your warranty and get the NHBC to investigate this. They will have records of their own inspections and know what was and was not done.
There is no harm in making a Subject Access Request to Redrow Homes. If nothing else it will waste their time in admin.
However, SAR do throw up some little gems from time to time when an inexperienced youngster includes a copy of something very revealing and therefore helpful for your case/complaint.
Regarding solicitors, you should ideally to see one near where you live.
As for a surveyor's/structural engineer's reports again a local would be better and probably much less expensive.
Make sure the surveyor is a member of the RICS and any engineer a member of the ISE.
However, your first course of action is to get a report from the NHBC and only if you dispute their findings should you go to the expense of obtaining your own report or taking legal action.
