Potterton fireside black beauty 35/51 back boiler the pilot light keep going out

Hello I hope someone can help with this one.
I have an old Potterton fireside black beauty 35/51 back boiler.
until this morning it was running fine. It flamed up fine and the heating/water came one fine. after it bit it turned itself off, and the pilot light went out.
I turn the starter vale to off and left it to cool down.
when I went back to it to relight the pilot light after about 20 minuets. It relight fine I turned the starter vale back to on. After about 20 seconds the was a hissing sound then a clicking sound and the pilot light goes out.
I gave it all a good clean with vacuum cleaner and air duster.
It does the same, the pilot light come one fire time very time. The flame look nice a blue and is one flame.
If I turn the heating and water on before I light the boiler it will flame up fine and run okay for a bit.
It looking like it get to where it will flame down in normal operation then after about 20 seconds it stars to hiss then the pilot light goes out.
Anyone got any idea what is up with it?
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