Replaced 'stat- still not right.

[jimmyred]

Following advice from this forum, I suspected that the source of the problems I had with my 03 1.8 petrol was a seized thermostat (temp gauge never moved, emissions light on, heaters poor, guzzling fuel and also misfire caused by burning rich maybe).
I replaced plugs last week and this cured misfire, although as week went on it did occaisionally do it again. I assumed that the rich mix was responsible.
Then yesterday I replaced thermostat (just valve, not housing, as temp sensor seemed to be working when I tested it by unplugging, despite damage to cable.)
This worked as expected- mostly:
The heater is now great, and the emissions light went off after a while. The coolant doesn't seem to be leaking, fingers crossed. The car just feels better, as it is now running at correct temp (I hope!) BUT, the tenp gauge is still not working. I had wondered if it was broken, before I changed stat, as I have never seen it move off bottom, unlike other people who said that it would sometimes rise slightly.
The old thermostat was definately knackered; I had to free it from housing with pliers.
To add to my bafflement the emissions light has returned, and I have also noticed a slight hesitation/misfire when changing gear. It's nothing like as bad as before, but still, something is wrong. I took one of the new plugs out today for a look; it was slightly black but I wouldn't have thought enough to cause problems.
I am just gonna run the car for a bit and see if these problems iron themselves out. I disconnected battery last night in the hope that the computer will "relearn" that thermostat is now working.
It all becomes a bit complicated if you follow cable up from sensor. As I say there is some damage close to thermostat housing, but the sensor is still working because when you unplug it the fan comes on continuously. Maybe the problem is at other end, close to the dashboard?
Any advice on this would be really helpful. It's doing my head in. For most of yesterday I was really chuffed that I'd fixed it, despite the gauge not working, but now the light and misfire have come back I am at a loss.
Fault codes read using pedal test:

1565: speed control command switch out of range

1482: SCP ?

1481:Fan secondary low with high fan on

0301: Cylinder 1 misfire detected

0115: engine coolant temp circuit malfunction

0567: cruise control something or other

some of these are old I think.