Overheat fault

I have a Saunier Duval Thermaclassic F24E PLUS (fanned flue combiboiler GCNo 47-920-37), which was installed less than 3 years ago. (I no longer wish to use that 'engineer'.
It was on timer, water and radiators - the red lock out light was activated by a code 05 fault - overheat fault. I managed to reset the overheat thermostat and reactivated the boiler, it was very noisy and produced a code 14 - over temp on primary water circuit.
Now the patter on trial is - I activate boiler, green light comes on, ignition lights the gas, boiler makes noises, temperature goes up rapidly, boiler ceases to heat at max temp (green light still on) then shows temp rising to 99 degrees C (also the pressure increases), then red lock out light and code 14 again. If I turn power off/on I am able to repeat this procedure without resetting the overheat thermostat.
From reading postings on this site, I think it is 'kettling' which could mean the pump (I have put pump on constant which made no difference), a build up of limescale/sludge or perhaps even the heat exchange itself.
I am hoping there is something simple and cheap that I have overlooked.
I have bled the radiators and there was a massive airlock in one of them. I would be very grateful for any help ... and so would my family! [/b]

how have you put the pump to constant??

Its really does sound as though the pump is not running.

any water left in the sys after venting

"""I am hoping there is something simple and cheap that I have overlooked."""

Often I am called by people who want me to come and fix their boilers. Some tell ME that its only something small! I ask them why they think that but never get any answer, its just wishful thinking.

There is no mention of system pressure in the posting, that would have been more reassuring!

I assume its a combi but there is no comment on how it behaves on DHW compared with CH.

S-Ds are not my favourite boilers!

Tony

thanks very much for replies

there is a procedure in the manual for pump operation 1- intermittent with room thermostat, 2- intermittent with burner and 3- permanently on. I assumed that if permanently on i would hear it pumping-there's a few ticks then silence.

i have used my external filling loop to refill the system an attempt to get rid of all the air that was in the radiators which i managed to do. I don't think there would have been enough pressure otherwise.

it is a combi and the behavior is the same on dhw and ch.

Nothing wrong with a bit of wishful thinking-if anyone did know any little tricks it would be someone on this site.

I'm not a fan of S-D's either!