mate i had a similar issue with my newly installed Vaillant over the last few months
f.28 on a vaillant means the boiler is failing to ignite - when you turn it on, do you hear a noise as though the boiler is trying to ignite?
could be a range of issues from the pipes being blocked (so no gas reaching the boiler) (but this is unlikely), and something has broken, like the gas valve or such like.
tbh, im not sure that releasing the pressure (even though your bar is almost zero) will solve the issue as the fault is non ignition
the fault code for the filling pressure going below 0.05bar is f.22 on a vaillant.
but if you have a filling loop on your boiler, give it a go and get some water back in the system.
maybe give vaillant a quick call first - they are not that efficient but you might get some knowledge you can work with
here is a link to the manual btw
http://www.vaillant.co.uk/stepone2/data/downloads/99/42/00/timeSWITCH_110.pdf