You can get an acceptable fixing in lightweight blocks, especially if the units are in a 'run' over the doorway as the units also pick up support from neighbouring units and their fixings.
However, I would use just regular blocks as they are cheaper, and do take plugs better. Not necessarily concrete as they are heavy and a bit overkill for an internal doorway (sorry WB!), and possibly more difficult to lay properly for a DIY job.
What builders often call 'breeze blocks', plasmore fibolite is what they sell down in Nottingham, just ask at the merchant.
Make sure that you tie the new work in a little too, galv angle brackets is the norm, screwed into the existing work.
Alternatives include timber frame and a plasterboard....
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