Karl is quite right, Cuppa, we would need a good deal more information about the construction to be able to give any meaningful advice. I'm also a little confused by "outside internal walls." It can only be one or the other.
What I would say is, it's quite unusual for cracks appearing internally, but not showing externally.
After the summer heat, if you are built on shrinkable clay, then subsidence is a possibility. But .......... that should show up externally in most cases.
Firstly, examine your leasehold agreement. Unless this has been specifically excluded, it may well be covered by insurance held either by the leaseholder or by your own policy.