Spring pressure valve on an open vented system?

I have an open vented system with Baxi bermuda inset 2 back boiler (10 years old), indirect water cylinder (gravity HW system), central heating is pumped by a Commodore 130-45 pump.

Close to the boiler in the hot water (unpumped) out pipe there is a T-peice and what I am told is a spring loaded pressure release valve connected via a short length of pipe. This is weeping (fills a jam jar within a week) so clearly needs attention.

Question is:
1) Why does an open vented system have a pressure release valve?
2) I can't find any similar replacement valves (and not sure what pressue it should be rated to as no markings on current part). Any suggestions?

Thanks

Wilko
October 2008