The pilot of my Potterton Kingfisher keeps going out??

I had a new thermo couple fitted by an engineer who is now ill and cannot come back. It worked for 2 weeks but now, in the coldest weather, the pilot light goes out immediately after releasing the control knob.
The pilot flame looks very small? Can this be adjusted?
Also the nut around the TC is loose (should be tight?) and the TC is also loose within this nut.
Any advice?