Internal solid wall removal between 2 extension

[2much2do87]

We have just started renovation work on a 1930s property we have just purchased. 2 extension have been added at various times over the years. We have opened internal solid walls between to of the rear extension. Can somebody advise on how to make the concrete base and existing cavity good. See images please any advice on the proper way would be great. Thansk

 

[yorksbuilder]

DPC across the whole area, in a U shape. Then fill the channel in the middle with concrete

 

[2much2do87]

Thanks mate I thought that for the base. And for the existing cavity wall should the dpc run up that on the inside of the outer brick skin when I do the concrete base. Right in the corner does look damp at the bottom of the cavity. Then insulate and block up?

 

[2much2do87]

What mix should I use to fill concrete base thanks