Car won't start

Sounds like a low cranking speed,
these need to spin over at least at 180 RPM,
though I like to see 200 RPM.
Low cranking speeds mean there is just not enough cylinder compression,
and as diesels ignite the fuel charge by the compressed air temperature it just wont ignite.
Tow starting spins the engine over a lot faster.
So start by adding a GOOD jumper battery to see if it spins over 180 RPM,
if it's still low then check the engine ground/earth cables/mounting points.
then it's suspect the starter motor.

I have a pug 206 quicksilver 1.6 16v on a y reg (2001)
It will crank over but it won't fire. I have checked all the fuses in the car and engine box (all ok) the fuel pump starts fine there is no spark I have put new plugs in it but still nothing been told it could be an immobiliser problem. Does the immobiliser run through the Ecu on this car?

Short answer is yes,
but What you need to establish is spark at the plug tips when cranking over.
If not then my first suspect just for reason of cheapness is the CRANK ANGLE SENSOR.
If it is working then there ought to be some register on the rev counter if fitted.

Check battery voltage and also check current draw when cranking as the starter may be drawing too much current when turning engine over.Check earth lead connection at gearbox aswell. Could also be a problem with engine speed sensor on gearbox or an injector fault causing very low fuel pressure on cranking

Thanks for suggestions.i 've tried it with battery booster and it turns over at same speed as usual but won't start so think battery and starter motor O K.
I 've changed fuel filter, old one did 'not seem bad.
Tried tow start again and it fired up no problem, runs as normal and rev counter works.
Still won't start on starter motor. Any other ideas?

As also suggested by 1/4 drive ,could also be a " lazy " starter dragging battery down .

it may be turning over at normal speed but you still need to check how much current it is pulling.