I had a chap come over today to boroscope the walls of our mid 70’s house to see the state of any cavity wall insulation. 
He confirmed that we do have existing blown mineral fibre insulation but it’s quite patchy and at quite a low density (seems to tally with the boroscope images he showed me but what do I know). 
He’s quoted £1200 to top it up (roughly 80m2 at 65mm depth)
Having done some research I’m none the wiser as whether to go ahead or not. Is it going to potentially cause problems ?  Am I going to see any appreciable difference for my £1200 ?

Any thoughts ? Thanks. 

Make sure the exterior brick / render is in good nick as if water gets in it can travel through the insulation across the cavity and cause damp internally.  Consider applying a coat of StormDry to the masonry on more exposed walls.