I have a Vokera Excell 80 SP. It may not be the same problem, but with mine, a lot of faults are attributed to lack of a porper service, which apparently for one thing means the diaphragms have not been annually replaced as a matter of course. Yet, I just had a service and this ain't been done, go figure.
As brand new in mine, the diaphragms fitted rotted when immersed in water, according to a particularly frank expert on Vokera boilers at the time. It went faulty in less than 3 months, and was repaired under warranty.
Note, the only time I ever got good service or repair on this boiler is when an actual Vokera Engineer attended - the rest of the time, with several different CORGI-registered plumbers, it's been loads of new parts, 80% of which turn out to be unnecessary, and a fix may or may not have been effected.