Boilermate SP - no heat to radiators

I've just tried switching the heating on for the first time since summer came, and the burner kicks in fine, but no heat gets to the radiators.
I've not really got much knowledge about these things, but have an idea that a valve that sends hot water to the rads isn't activating?
I've pulled the front cover off our Boilermate SP and figure it's one of the big motorised valves perhaps sticking from not being used for a few months. Can anyone advise which? Can the screw slot on the front of the valves be turned to an open position?
Sorry if I've got the terminology wrong!

Or if anyone has a better suggestion of what the issue could be! This is just my uninformed guesswork!

I'll happily call a plumber, but typically it's a weekend now, and I'm working every day til Tuesday, so I thought I'd find out if there's a fix I can do myself to get the house warming up!

Also, the mechanical clock timer unit on the unit doesn't work - it ticks around, but doesn't turn on or off when it's set to. Is this an easy part to replace? I only use this timer for water heating, the rads are left set to 'constant' and activated from the digital thermostat downstairs.

Thanks

Hi, the three port valve in the system should have a manual override lever on the side of it which can be moved over to the various positions.  If you can see the make on it have a look on YouTube someone will probably have put a video up about it, if not there are plenty of others on three port valves that give you an idea.  A common issue after a summer of no heating is that the central heating circulating pump can get stuck, you can hold something against the pump like a large screwdriver and then pop the other end to you ear, redneck stethoscope, and see if you can hear it running.  If it is just stuck this can be checked and remedied fairly easily, this is a video showing someone doing just that.  https://www.youtube.com/watch?v=F5sMNa8bjrA - Be aware that a small amount of water will come out so as in the video have a towel ready, also the water may be hot so again take care.   I am guessing the timer is a Grasslin model, these plug in so are straightforward to swap out, this is a link to Keep The Heat On they have them, https://www.keeptheheaton.com/products/44530-gledhill-xb215-mechanical-time-clock  Once you have a replacement you should be able to see how it fits etc..  As always if you are not sure/confident that you can work safely get a professional!