As ian has said really,What tyres are you using or how old and how much tread is left on them.

used to lock mine up quite a bit if they were cold in my old XE

You sure its related?

Colder mornings, colder tyres and more effective braking could just mean you've found the limit of the tyres.

If it is a fault with the brake lines it might be that its not the same both sides in which case an MOT ramp would probably give you brake force side to side. If thats in tolerance then I'm not sure. Original flexis back on and drive that cold as well perhaps.

I don't have abs

Sounds like a problem with the abs.

Moring chaps got a question to see if anyone has or had the same problem. I have owned my Corsa b 20XE now for just over 6 year, I have fitted braided brake lines a couple of month back and I seen a big difference in the braking system miles better and a better brake pedal. Now my problem is I've noticed on the colder and damper morings if I brake ever so slightly it's fine but if I brake that extra bit more my wheels lock up its never done this before only since I have fitted the new brake lines anyone had the same thing happen or know anything to try to get around it as I'm not looking forward to the winter lol