Hi Simon,
I recently bought a keg from that smalldogtrading guy. Unlike some other sellers, he didn't slap on any hefty surcharge for postage to N. Ireland.
The corny, like you say, had no pressure relief valve but that's no biggie; you can just use the gas in valve instead. The keg came still pressurized, so no problems there; seals were good. The only issue I did have was when I opened it for first use there was the tail end of some soft drink syrup in it. Again, no big thing; I just washed her out and flushed her through as I would have done anyway. I've had 1 brew through it now and all was good.
The delivery was pretty quick, too.
Hope this helps,
-Barry