Expansion vessel's perished in my system too, but since it's a one floor flat it doesn't need to go up to a significant pressure to pump the water round, so except for midwinter I can run it low without having to top it up very often at all (and maybe only a couple of times during the winter).|
The pressure release valve in mine is in front of the boiler, a little above the top of it (the pipe leading outside runs over the boiler top). No reason it has to be near the boiler, other than convenience for engineers when they're installing everything for the first time or refilling the loops, as far as I know. Look for the outlet pipe on your external brickwork - you should see two of about the same diameter - one will be your toilet overflow, and the other the heating overflow from the release valve. Since you know it pours on to your extension it shouldn't be hard to spot. On mine the top is red, and it has a little gauge on it, indicating the pressure at the top of the system.
My boiler's a Potterton, I think, and the original mainboard cracked soon as you looked at it which took a number of engineers over the years to finally diagnose. Since that was replaced the boiler's run sweet as (for under 300 quid as I recall). I'd be a bit dubious about replacing it with a modern condensing one, because of issues with the condenser corroding and needing regular replacement.