The best way to test, is as advised. Shut down all the heating for 24hrs and then run the tap, if it's cooler/cold than normal then that's your issue and it will be thermal transfer and then it's time to trace the pipework.
I really wouldn't suspect it's expansion as 30secs worth is a fair length of pipe to be contaminated, that and there's a new combi valve so the check valve should be working fine.
I really wouldn't suspect it's expansion as 30secs worth is a fair length of pipe to be contaminated, that and there's a new combi valve so the check valve should be working fine.