Peugeot 306 HDI Turbo 2 ltr Y reg won't cold start?

Above freezing temperatures, my 306 hdi starts, theres a bit of a knack to it, but it always starts. (the knack is, it i continuously crank it, it won't start, but if i give it quick bursts on the starter, it starts). The main problem is below freezing, i turn the ignition on, wait for the glow plug light to go out, turn the engine over, it bursts into life for one second, then won't restart. It will start eventually with a jump start, but it takes a lot of goes to get it going.

I've replaced the glow plugs and control unit, the previous owner replaced the injectors, a brown relay? and the temperature sensor, he also had the ecu remapped after a wire came off somewhere and shorted. He also checked the low pressure pump.

So , to recap, when it's warm it's starts first or second turn of the key, when cold it doesn't, any ideas please anyone?