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I have a cavity wall which is accessible from the loft. Ie it is an open space. I have noticed how warm it is and know I need to insulate it. Can I simply pour in expanded polystyrene balls, or should I just stuff some loft inslation woolly stuff in at the top?

now i may be old fashioned but there is a reason for having a cavity and thats to allow air to circulate and keep the bricks and timber from moisture,it is in effect allows the building to breathe
me id leave it alone and look for otherways to keep warm
bob

I have a similar situaton with our kitchen extension although the flat roof is covering the top of the cavity. I believe the normal way to fill cavities is by pumping in foam or strands of mineral wool, so your best bet may be to contact one of the companies who do that. I think the risk of dampness across the cavity depends on the exposure of the wall to wet weather. i.e. is it facing southwesterly or high up a hill for example

your idea of filling with vermiculite is very sound.Make sure that it wont block any air vents.air bricks.etc.You can usually tell if theres damp penetrating the outside wall into the cavity by smell etc.Mostly in non victorian houses the cavity is dry.A cavity with air circulating is disastrously cold.The advantage of vermic is-if a dsaster happens you can suck it all out again.You cant do that with wool or foam.Its aggod idea to seat top of wall once filled with maybe fire resistant polystyrene which you can cut to fit tight in the gap once yuve filled it.Or ram some mineral wool in there. Funnily enough im doin same but got to find way to blow granules in..Congratulation on your theory

I have been informed that insulation of wall cavities can not be done if the building is of stone construction, this is due to irregular internal profiles of the stones which may prevent equal distribution of the fill, this could cause cold areas on internal walls creating damp etc. brick walls are ok to do.
there is a national eco scheme where you can get it done cheap or even free, log-in to 'warmstart'