I had a leaky hot water tap on a shower, so I replaced the plastic
rubber O ring bit inside and this stopped the leak.

But two weeks later (and possibly unrelated) the same tap when you
turn it on puts out no water, it takes about 1 minute for water to
start coming out.

I am guessing I have a leak in this line. But the pipe goes directly
through a solid double concrete wall to the hot water system so there
is no access what-so-ever. Because its on the hot water line I can't
see my meter moving and I haven't noticed my hot water service filling
unusually.

Does anyone have any ideas on how to figure out what is going on?