pilot light issue... please help?

Hi,
Can You help me please ?

The Pilot Light on my Puma 80 keeps going out.

If I switch on my Central Heating and run it for about 30 minutes or more, and THEN decide to run the hot water as well, to wash the dishes or take a shower for example; as soon as I turn the hot water off, I hear a CLICK and the pilot light goes out.

This happens EVERY time I run the hot water while the central heating is switched on.

The pilot light is perfectly fine if I just run the central heating on its own, or if I just run the hot water on its own.

But as soon as I have them both on at the same time, the pilot light goes out.

Any Ideas ?

My system pressure is fine, between 1 - 2 Bars, there are no leaks of any kind and no funny noises.

I had the thermocoupling assembly replaced by a CORGI engineer last month, but it didn't help much, the problem still occurs...
I have called him back, but he doesn't know what else could be the cause and says I will probably need a new boiler... which I can't afford at the moment...

ONE LAST THING TO NOTE THOUGH:

When I run a hot water tap for more than 1 minute and then turn it off, the boiler continues to be fired-up for about 3 more seconds before it stops and it is THEN when the Pilot Light 'Clicks' and goes out.

Please, Please, Please help.....

Thank You