Hey there
I sat that exam as well, mmm yeh there was a few quirky ones thats for sure. For the let by test I assume that referres to the gate valve or ball valve holding gas pressure. Ie if it were "letting by" loosing pressure this test would confirm this?.
The thermal nut regulator question was I guess for me, I know what it is, but its purpose is tricky.? How did you go on the self igniotion sketch? Full marks lol? I gave it my best shot, but yeh not sure too!

A service valve "Let By" test - the purpose of a service valve let by test is to ensure that the service valve does not pass gas into the installation when in the closed position.
Procedure:
Ensure the service valve is turned off
Fit test piece to the meter test point
Connect apprpriate gauge to the test piece
Slowly open the service vavle to pressurise the pipework tot he working pressure
Close the service valve to isolate the gas supply
Bleed gas from the test piece isolation valve until the pressure drops to 50% of the working pressure
Shutt off the test piece bleed valve
Wait for two minutes (incoudes temperature stablisation)
If the pressure has not risen then the service valve has passed the "let by" test
If the pressure has risen then the sevice valve has failed the "let by" test and the network operator shall be notified.  the netowrk operator shallr epair or silate the servive valve and the service valve "let by" test shall be repeated
Remove the test piece
Record test results

Kozan clip on reg for the third type, let by test proves the integrity of an isolation valve by testing down stream and below inlet pressure of the valve and watch to see if the pressure climbs. Thermal nut melts in a fire and shuts down cylinder.......but hey f**** it I want a pair of those jeans man, bang on suiuan, dem pants are mean