I'd thought I'd follow up with my experience.

I did a large 100% water change, switched off the lights and covered the tank with some large sheets of black cardboard (I knew they would come in handy one day!) and added a old 2nd internal filter I had lying around.

I added Stability every day and 2x dosage of Excel on the 2nd day and left it for 3 days.

After the 3rd day I removed the sheets and switched the lights back on.
The tank looked ok. I did a large water change and removed any damaged or algae touched leaves. I fed the fish.

On the 4th day we had our 1st sunny day of the year. The temperature in the tank rose from its more usual 22-24C to 27C. I put on a large fan next to the tank to try to reduce the temp.

But I got up on the next morning to find 6 dead fish. 4 corys were dead and 2 zebra loaches. I also found one of my Dwarf African Frogs had escaped and died all dried up.

I did water tests.
Ammonia 0
Nitrite 2ppm
Nitrate 5ppm
ph 6.4 (usual)
GH < 3dh
KH very low 3d or lower

My clown pleco and golden bristlenose were still alive.
The white cloud mountain minnows and glowlight tetras were almost unaffected. Just sitting low in the tank that day.

I added a dosage of Prime to get rid of the toxic nitrite. But it was only 2ppm and the tank was aerated throughout and the surface did have some water movement due to the filter.

I did the tests using Tetratest sticks and a API freshwater master test kit.

No diatoms.. But at quite a cost.... I really liked my zebra loaches...

Yesterday I tried to feed the 2 last corys and zebra loach and they're just not in the mood. probably all stressed out.

This morning their behaviour is looking a bit more normal.

I guess I'll never know why the fish died on mass. ph shift perhaps or something toxic from the old inserted filter?