Not a lot of immediate help but well worth a read:
http://www.diynot.com/diy/threads/worcester-bosch-35cdi-ii-losing-pressure-constantly.259268/
https://www.worcester-bosch.co.uk/support/troubleshooting/faqs/all-faqs/the-heating-system-is-constantly-losing-pressure-what-should-i-do
It may be worth checking the expansion vessel is properly pressurised and that the membrane inside is intact.
Also check the PRV -0 the vent pipe to th eoutside world. many suggest putting a paper cup or a plastic bag over then end to collect any slow discharge.
XXX
January 2017