Potterton Puma 80e pilot light slow to stay on?

Dear All,

recently I've found that after running a bath the pilot light goes out and when trying to relight it I have to hold the gas control knob in for longer and longer times-if i release it too early the pilot goes out.
With this cold weather it's a disaster waiting to happen!
Over the last two years I've had all the circuit boards replaced ( apart from the front one that does the dials), the thermostat and the pump.
I suspect the boiler thermostat again- it's about two years old.

Any suggestions, especially on how to remove, and check the thermostat would be greatly appreciated.
And lastly,
Merry Christams!

with thanks in advance to all who help,
Marta.