Guys, just wanted to report back - my old water heater now seems fine. 3 weeks without a re-light!!! Normally it'd go out every 2-4 days!
So, worth me stating the fix, should it help someone else - really a lesson in trying the easiest thing first....
So, as previously reported, the new thermocouple didn't improve things. I then had a blockage in my main plumbing line, so had a plumber out. While he was there, I asked if he could take a look at the water heater. I showed him where I'd replaced the thermocouple, he said everything looked good, and if it wasn't working at that point, i probably need a new water heater. He also thought it was unlikely that the missing viewing glass was the problem, but he said he had some aluminum tape, he could stick a bit over the hole and see if it helped. It couldn't hurt and would only take a few seconds to do. So, he did that, and it's not gone out since!
So, as unlikely as it seemed, i guess that piece of glass not being there was the critical element for me. Very happy now - think i got a bit lucky there, as i was already researching new water heaters! That $80 call out for the main line blockage has saved me some money!!! thanks for all the advice!
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