I have had the same problem on a Suprima 50.
Probably the same board.
i had it installe about 10 years ago and after about 2 i started getting the problems. called a boiler guy in or a service he said the pcb need to be replaced.
I checked on the net and decided to fix the pcb my self - i'm a technician-
The Boards area bad design. The solder joints can go dry and just touching them up with fresh solder fixes that.
I found that some of the tracks on the pcb were lifting and going open circuit. Specialy between two of the diodes on the board there is normally discolouration around them on the board. due to heat.
i was able to solder on a single core piece of wire to by pass the faulty circuit.
This has solved my problem. It started acting upagain recently. i found that the solder on my repairs had gone dry too so touched it up again.
Q----Another strange symptom that has started happening a couple of times is that the Boiler has on a couple of occasions come on all by itself late at night when the heating and hot water timers have not even been set to be on.-----
A:on some systems there is an over riding thermostat that will override everything and turn on the boiler if the temp drops to freezing-- check that its not set too high.
Q----Also, the redirector (i think that is what it is called - it is the little black box under our water tank) occasionally makes a buzzing noise when the boiler should be on but won't come on.----
A. One of the symptoms of the dodgy pcb is that the relay on the board is not gettting the signal to switch off the pump. it maybe that.
Q------Can anyone advise what to do please? Am I looking at another costly engineer visit?------
There are options. If you are anyone you know is handy with a soldering iron and can recognise faulty circuits and soldering then have a go.
Be careful though--- you MUST switch off the main power to the boiler to fiddle with the pcb.
Its best to remove it to work on it.
Cheers
Hope that helps.
Rory O'Gorman
January 2008