It depends on who is inspecting the trees and why. NTSG identifies diferent levels of responsibillity and how to discharge the duty of care. For example it is fine for a private resident to monitor his own tree through informal observations. i.e. just keeping a general eye on tree condition while cutting the lawns and then calling in a professional when required.
The same document discusses the responsibillity of a rural council in section 5. It states that highway trees are inspected proactively by highways officers in the first instance and any concerns are relayed to the tree officer who has a level 3 arb qualification for a detailed inspection. I see no reason why this cant be applied across the board. If a gardener inspects a tree and cannot decide what to do he should seek professional advice and in my view this means level 3. OK it doesn't specifically say this but its about what the courts consider to be reasonable if something goes wrong. There is also a bench mark qualification in Lantra PTI for professional tree inspection. This again would be reasonable in my view.
The AA reg consultants list requires that you have level 5, the ICF is level 6 i think, and CAS is level 4. These are all relevant as you could use them to make your case to the judge if you wanted to discredit someone.
If the prosecution were to say this chap has a level 2 qualification and presenting himself as a professional or expert, yet all these organisations say at least level 4 for professional membership, NTSG discusses L3, plus if you wish to gain the Lantra PTI the pre-requisite is L3 arb qualification!!! It would be very difficult to argue and put up a suitable defence.
There is no legal requirement but there are plenty of indicators out there to say what you should have. As someone already stated 5837 and subs investigation are at a higher level and i dont think you would get the required skills at level 3. This is based on my own experience of going through L2, L3, L4, and L6 to see what is covered at the different levels. Some may have a different experience so thats just my view.
Cheers,