Thanks to @presscall teardown of his Apex was able to find the brittle o-ring causing my Apex pump to leak. Yes, it's got the crack but it wasn't leaking there, rather from the fuel valve on/off. Replaced this with a 005 viton type (used in the 550 also) and leak solved! Simmer to full blast on an Apex 450A700 (04/95) burner. John
Thanks @Daryl It's crazy, everything on that stove set is aluminium, pressed metal or brass (even the level adjuster is brass!) but the pump housing is a crappy plastic. A real shame because if that housing was metal, these things would just go and go. Simple user serviceable parts too. I'll keep using it until I can't.