Any extra steps before filling holes in screed floor after plumbing work

We have had some modifications done by our plumbers to the heating pipework in our screed floors for a number of radiators and I am interested in peoples thoughts for replacing the screed and damp precautions.

The holes vary in depth from 10 to 15cm, total about 9 metres and the house is late 1970s. There's no sign of DPM between the screed and lower concrete part of the floor so I am presuming there is DPM below the concrete. As the screed crosses the DPC in the wall as per the picture below I am wondering if we need to consider anything for damp due to this bridging although we've not had any issues since we moved in.

The plumbers are intending to just put a sharp sand mix in after putting some celafelt wrapping around the pipes (as an aside I am considering at least doubling this as it looks too thin). For the damp protection I am wondering about coating the wall below floor level with a liquid DPM and doing nothing with the screed/concrete other than priming it with PVA and another tacky coat to improve bonding prior to the screed being laid.

Any thoughts or other suggestions?

 

There's been a bit of discussion on protecting the copper and the plumbers say Denso doesn't allow movement even though its good on the corrosion aspect. However they're didn't even consider pipe protection until I mentioned so they may be going for the quick and cheap route.

The pipes were previously protected with a layer of loose fill material which actually has done a good job for 40 years. The problem I can see with their celafelt suggestion is once wet with screed it doesn't look like it will allow movement either and its inner plastic coating doesn't look that protective from corrosion unlike Denso.

As I always have thick plastic DPM in stock I was wondering about covering their celafelt with that before they fill in.