dpm needed to lay vinyl on new screed?

Give the lot a screed, you can get away with feathering the screed out away from the ply eventually bringing it back down to 3-4mm but that's with someone who knows what they are doing. You won't notice.

.... seeing as you are only having click and not stick down I'd feather joins on the plywood then feather out about 50cm onto the concrete then screed the lot from the ply.

You are having a floating floor with underlay, you'll not notice the few mm difference if done correctly.

 

That's if there was a 6mm difference but if it is 3mm then simple feather join and give the rest a 3mm coat

 

Thanks, great advice. I am thinking that my best way forward may be to get a pro to do the levelling (as that seems a bit beyond my DIY skills) and then I have the subfloor I need to lay the click stuff (which I am confident in doing).

 

Yes, you are right. That's the great thing about forums like this; we DIYers can learn how to do stuff and, more importantly, find out what we can't do.

Thanks all for the advice.