Oil fired boiler, central heating without pressure - what to do?

Bit of background, this is a holiday home that I think has been without central heating for a few weeks at least, the house has that feeling of cold in it despite the heating having been on a timer and normally working fine. Oil levels are fine, boiler is turning on but appears to turn off after a few mins. Rads downstairs do appear to heat up a slight amount but then it disappears.

Note - I don't know if this is anything related but our outside tap sprung a leak over the last few weeks as well and was constantly running but I've fixed that this evening.

Had a look at the central heating system and the pressure looks to be completely gone - I've read online that I can open a valve and get the pressure back up, I've also read that there's generally two values to open, but I can only find one and it's open. See the photos, there's another 'valve' on the inlet pipe but I don't think it's really a valve, twisting it just lets water out. So, I know water is getting that far, but the pressure is not increasing. I've bled all the rads, they bled for a long time, some upstairs just stopped without any water coming out.

Any ideas what I can do or where the other valve might be?