Domestic hot water tank overflowing, new ballvalve, new cylinder, new mixer under bath and new reducing valve, still overflowing

When you say "overflowing" - do you mean Pressure Relieve valve is leaking ?

No mate, mixer and reducer fine, tank filling and running out of the overflow pipe

Does it look like this ? See image below...

No pal, I think the cold feed is by passing through the mixer, but I've put a new one one so??

I am sorry, I don't understand your terminology... Can you snap a picture of it and post it ? Thanks...

No problem my friend having a go again tomorrow will let you know how I get on cheers