Cavity insulation or not.

I have the opportunity to get cavity wall (bead) insulation installed at a reasonable cost.The house is a 2 storey detached with attic conversion.House was built in 1981/2 and no cavity insulation installed, all exterior walls have plaster board with 28mm insulation backing attached to walls by dot and dab method, all windows D/G(4 yrs old) with trickle vents, all bathrooms have extractors fans linked to light switch.Attic insulation is 300mm.No other room vents in house except in utility(dryer)kitchen( Hood)and two blocked fireplaces.

Installer has said I would be ok to install bead,he has produced agrement cert(regarding ingress of water in exposed areas) that certifies bead is suitable for rendered walls but not brick,my exterior walls are rendered.

I have a few questions, hopefully ye can offer some guidance.

1.Will the bead insulation make a difference, are uninsulated cavities an absolute no no nowadays?

2.I am worried about condensation, do I have enough ventilation at the moment?will the insulated board currently in house impact on cavity insulation as regards to condensation?

3.One of my gables gets some s/w showers and is more exposed to weather than the others which are more sheltered(I am in urban location so some shelter from neighbours) should I be worried about water ingress?

Any guidance or tips very much appreciated.