Just for a follow up for anyone reading the thread in the future. As I had suspected the leak underneath from around the tank to pipe spigot entry was not found to be a seal to toilet (it would have leaked inside then) Having removed the toilet the spigot needed replacing as fractured and leaking. Whilst the spigot is screwed to the floor it is also of a smaller diameter than the hole in the tank! So in order to stop a full tank from overflowing the area around it was filled by sealant when built (about 1/4 inch gap!) This then makes it very difficult to get out. It is possible also for this leak to have come from a failed seal around the spigot to tank.
Parts are
Dometica floor 3 inch flange item 385345892 $9.19 Amazon
Dometica flange seal item 38531006 $20.13 Amazon
I also found that the original build had the securing bolt of the toilet flange to raised extension from the floor done up at the back and difficult the get to. When refitting turn the clamp around for easy of access fitting and if you have to dismantle again :)
Hope that helps some one else!