I had another roofer out last week to take a look. He spent quite a bit of time looking at the roof. He came to the conclusion that the roof was unlikely to be causing the ingress and that the problem was likely coming from elsewhere.
A couple of days later, I looked at my neighbour's house (which is identical to mine, only without the extension) and noticed he has two vent pipes where my extension room is built over.
In the picture below I've drawn, in blue, the outline of my extension on my neighbour's house:
The white plastic pipe on the left is an overflow from the bathroom toilet and the one on the right (I believe) is some sort of overflow from the hot water tank.
In looking at my bathroom toilet and lifting the hood off the tank, I saw that it has been trickle overflowing. I immediately cut off the water supply to this toilet and within a couple of hours, a wet patch on my extension ceiling began to dry out. You'll see from the above pic that the white plastic pipe is just-on or just-below the roofline of my extension.
So I pulled back the lead flashes to see if I could find the 'missing' pipe. The yellow circle indicates where I believe it should be:
A bit closer:
No pipe to be seen, it's just out-of-sight, by the roofline.
Now, if you check this thread:
https://www.diynot.com/diy/threads/water-pump-stopped-in-central-heating.575440/page-4
you will see that I installed a new water pump in my central heating system a couple of days before I noticed the bad water splurging on my ceiling.
Now this is where my story becomes complicated. I believe the thermostat on my hot water tank was faulty as I had to wrench it up to full in order for it to heat the water, albeit to scalding hot. So is the reason for the main splurge of water in my extension ceiling because of the new, better working pump, operating with a faulty thermostat, causing it to eject water from the external 'exhaust' pipe, right onto my ceiling?
I replaced the thermostat on Saturday and tried heating the water (thermostat set at 60 degrees) and the system worked well; in fact the bath actually filled up quicker! More importantly, there was no water ingression to my extension ceiling.
Have I solved this kinundrum? Did the builder's simply attach the extension, ignoring these vent pipes?
The nuisance fact is when I installed my water pump there was bouts of very heavy rain for that day and the next couple after it.
The next bout of heavy rain will reveal all...