I have had a Vokera Combi Boiler and a Mira Response shower for approximately 5 years now. Both worked brilliantly initially however, shower now alternates between hot then cold periodically. Have bled the system but this hasn't made any difference. Can anyone suggest what the problem may be / how to rectify this?
You do not give the model of your boiler, however from its age I guess if you Google combinations of 'vokera flue venturi' & 'temperature cycling' you will get the answer.
I suspect that you will find the same hot/cold DHW problem trying to run a hot bath. You would not notice it on the central heating.
The air pressure switch is telling the boiler there is marginal/insufficient air flow so it cuts the boiler out; the cause is grime building up on the flue venturi. The fan, the heat exchanger and the flue venturi need to be cleaned, and the rest of the boiler is probably overdue its periodic service. If you are a 'competent person' and its the boiler in your own house you could download the manual from the Vokera website, and DIY.
Otherwise find a local Vokera service specialist (ask Vokera) and get them to do it. There is a small chance it could be scaling in the DHW heat exchanger, but my guess would be on it just needing a competent service job. Probably 150 quid or so, it may prevent other problems associated with missing occasional proper maintenance for a while from cropping up (expansion vessel etc).