patching blown plaster with thistle hardwall ?

You can use hardwall, or any of the other thistle backing plasters - browning, bonding etc

You need to damp down the brickwork before starting, and where the patch ends and the existing plaster begins, give this a coat of PVA, so that any backing plaster that gets onto the existing wall is well adhered.

Skimming sticks to almost everything, but the existing plaster will need a coat of PVA prior to commencing.

NB. New backing plaster like this should be skimmed the same day, or the next day at the very latest....any later than that and the skimming dries up and cracks off almost immediately.

Yes, unless the wall is damp, in which case you can't use any gypsom based scratch (under) coat. It might have been done with sand and cement originally for this reason.

I would phrase that a bit differently (but with same end result)... Finish coat doesn't actually stick very well, except to the previous plaster or sand/cement coat providing it's still wet (and ideally almost but not quite fully set), and to tacky PVA (so you can plaster anything if you paint it first with diluted PVA and let it almost dry before plastering).

Use whatever you've got. There's no essential difference between any of them, and anything will crack if the surface underneath it is too porous. Pva the wall, and then the undercoat when it's dry, and you can't go wrong. Always worked for me anyway.

This is the old argument about whether you should allow the pva to dry or not. It's never made any difference IME.

Thanks everyone for the advice. i'll crack on with the hardwall stuff then. (damp areas already done with sand/cement & waterproofer) One last question, if i don't have time to skim within a couple of days applying the new backing plaster, do i just treat it the same as the old plaster before skimming - weak pva to seal, let it dry, second stronger coat of pva and skim while still tacky?

Thanks again

Tacky doesn't matter IME. Seems like nitpicking but, if it means committing yourself to plaster at a certain time, it can be a pain. Pva works by reducing suction, which it does equally well when dry.