What might be causing leaks in my roof and ceiling?
In the end, the roofing company had ceased to exist which was why it was difficult to get hold of them.

I got a reputable building company to sort out the cracks in the walls and weatherproof the render - they did an amazing job and they even found a fault in the way the new roof was fitted... and fixed it for free.  The fault was that the new roof was just a new covering (although well fitted and looked good to the untrained eye), but it required some lead flashing (like a skirt) to force water run-off from the walls to run onto the roof and towards the drain/drainpipe.  Without the flashing the water was allowed to run between the roof covering and the wall itself, hence leaking underneath in particularly wet weather.  The issue never returned.