I can't quite see it from your picture, but I reckon your first port of call ought to be the system pressure gauge. I have a Worcestor Bosch - similar vintage to yours - and on mine, it looks like this:
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If yours is basically the same, there'll be two pointers - a red one and a black one. The red one shows you where your water pressure needs to be, the black one shows where it actually is. If that black needle is way below the red one, you need to let more water into the system. Somewhere close to your boiler you'll find the water pipes connected to it. Mine look like this:
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Find that tap - yours will be similar - and turn it anti-clockwise, while you watch the dial. The black needle should start moving up. When it's level with the red one, or just a tad below, turn the tap off. (Do this carefully...the pressure can change quickly.) Then use the reset button, as per irw's diagram above, and if you're lucky, you might be in business.