Had a Valve job now the engine is missing

Hi,
I am stuck in a problem which is regarding a fault with my car.
Few months ago, I had my camblet and cylinder head changed by the Halfords after cambelt was snapped and made some damage to the valves and cylinder head (according to the halfords). Then after a month after I started to feel bit engine shaking feeling in my car but I thought it may be due to the due car service and for services I took it to KwikFit, but very next day, I saw that Engine Light came on and after some time just gone off.

I rang KwikFit and told them about it and they suggested me to pop-in and they will have a look at it. After connecting the diagnostic tool they advised me that there is some problem in the emission but when I told them about cambelt and cylinder head change they told me to better is to take it back to Halfords and they carried out some work previously. I took it to halfords where they told me leave the car for further investigation. After having it for a month they told me that there was one sticking valve inside the cylinder head and it happened due to the excessive head in the cylinder.

And they also told me that, cylinder three was running lean which caused the excessive heat and damaged the valves. But they were unable to find the underlying fault with the car and advised me to take it to VW for further investigation, which I did. But after the one hour labour VW told me that there is no problem with the car whatsoever and everything is running as per the company's standard. But halfords is not happy with their finding and not ready to take the responsibility of their part and told me if I could not find actual fault then they will not be responsible for any fault or damage occurs to the car.

Now, I want to know what can cause that excessive heat in the cylinder (or this is only the assumption of halfords supplier). According to VW engine is not running lean then how did halfords came to this conclusion. Halfords supplier also suggested there may be some problem with the catalytic converter or less oxygen in engine. Halfords is also saying it may be due to faulty ECU or faulty fuel injectors. But problem is no one sure about the problem and only making assumptions. If I just follow halfords assumptions then VW is asking me few hundreds to investigate in detail.

Can someone please suggest me anything about this fault and any specific test that can prove that there is no problem with my car because currently I cannot drive it because I am afraid if something happens then no one will take the responsibility and I will have to bear all the cost.
Please help me.

Regards,
Vik