My Danesmoor 26/32 boiler keeps repeatedly locking out?

Does anyone have any ideas how I can fix my boiler which keeps locking out? On reset the boiler fires for about 5 minutes. I have to wait a few minutes before pressing the reset to successfully fire up the boiler again, otherwise the lock out is almost instant.
There is plenty of oil.

The same tank supplies the Aga and we have been putting in additive for the past 8 months.

The water supply is from the mains, not a storage tank and work has recently been carried out by the water authority. It may be a complete coincidence. I have noticed the water pressure seems to change when running a hot tap or trying to shower.