Off peak not heating

Hi, I have pulsacoil a class boiler. Before christmas the boiler has overheated(boiled the water in the top tank) and stopped working. We had professional person come to have a look at it and he said the off peak relay needs replacing which he done then and there.that has got rid of the fault. He said to leave to boiler on to let it heat up. May i mentioned he found incoming cold water sensor( thin wire) not connected properly which he fixed.4days later still no hot water. When I checked the temp on the control board it was around 35-40 and didnt go up unless I used on peak boost. We called the guy again to come see what else is wrong with it but suddenly the boiler started working on off peak so we cancelled the call out. It worked for 3-4 days and now not heating the water again! There's no faults on the control board. I've got "on peak supply present" "s5 sensor bottom calling" "off peak supply present" and "off peak heater on" and now and then "DWH pump on" also bottom left light flashing which means the system is running normal. When I press S2 few times it shows FF which means fault free.
I'm not sure if I'm on economy 7. I don't think I've got timer either as I tried to google to find out what the timer looks like and can't see anything like that on my boiler. I've got two switches on the wall which is on and off peak heaters.
Can you please help me... I'm just puzzled as to why it took boiler (off peak) 4days to start working after realy change and only worked for few days and now not heating the water again. May I just mention that when it did work it heated the water up to 78-79 and then switched itself off so obviously didn't overheat this time.

Hi
This could be some fault with your OP electric supply . But both of your switches on the wall has to be on. One is the switch for 24 h electric an !!! also used for your Booster, the other switch is for your OP electric and also has to be on all times. to work with the Boost you have to press the boost baton on your Pulsacoil a class, a red neon comes on and stays on until the cylinder is full heated at the top half.( red neon comes off). The timing for the Off-peak is direct operated from your electric meter. so no separate timer on your boiler.
I hope this is useful for you
regards Mario from Mario's Boiler Service

Thank you for your reply mario.

After the man has changed the relay I had the boiler on (both switches on the wall on) for couple of days but it didn't heat up the water at all...if I wanted to get some hot water I had to switch the bottom button on the wall off then press boost button (the red light came on) otherwise the boost didn't work, after the on peak boost heated up to the max temp it switched itself off and I then switch the button on the wall back on, after a while the water would cool down and had to do the same thing again when wanted hot water..then after five days boiler started working normally with the both switches on the wall on. It worked for few days then stopped working again last Thursday, didn't call for repair coz wanted to give it few days to see if it will start working again, in this time I used on peak boost (as described above) when needed hot water in the evening, suddenly today boiler is working normally with both switches on the wall on and it's keeping its temperature up so far.
Wonder how long it will take to stop working again...
I thought the timer if there was one was set to come on once/twice a week for few hrs or something like that as couldnt explain why the boiler would work for few days then stop and then work again and it doesn't show any faults...

there could be a loose cable on the immersion heater, or the relay ( as you say was replaced) is faulty. But is tricky to say without faultfinding on your Boiler, you need at least a multi meter and a clamp meter to do some testing. could be also the store sensor faulty ( 4 core sensor at the top)or pcb . Sounds like Of Peak is there course you have to switch the wall switch off, to get the boost working.