In uk.d-i-y, Bazzer wrote:
Hi, only just found this group.

Welcome to our little corner of virtual paradise ;-)

I expect my question has been asked many times in the past

It has - try the Advanced Search at groups.google.com, restricting
the group to uk.d-i-y and using keywords like "soundproofing".

I have been told of a heavy type of rubbery flexible sheet type of sound
isolation material.

"Heavy" is part of the right answer. While cutting down the higher
frequencies isn't too hard, cutting down the booming (or worse, floor
shaking!) bass is damnably expensive, and challenging; you need all of
(a) a lot of mass/weight, (b) a lack of rigid coupling to next door's
structure, and (c) near-obsessive sealing of air gaps. Nearly always
cheaper to move in the long-term, to rearrange the way you use your
space in the medium term (maybe you can move a bed across the room, or
take to sleeping in a different room if that's the main issue?) and
short-term turn to suitable mixture of headphones, earplugs, and drink for
yourself ;-) Eggboxes are quite useless to cut down on transmitted noise,
in case someone tries to sell you some ;-) And it's easy to get obsessive
about the issue, and find yourself in the region of diminishing returns
(so, the first few hundred quid you spend gets you say 80% reduction; then
the best part of a thousand gets another 70-80% of the remainder, which
is only 14-16% of the original level but you've become used to the lower
level now, and so on and on!).

Sorry not to be more cheerful!

Stefek