I own a one bedroom house with its own front door and integral garage. Once in the front door, to get to the one level living accommodation you climb the stairs. My house is the only one like this in the development. Under a large part of my property are 2 parking spaces for 2 neighbours who rent. I do not know who owns the properties. One neighbour repeatedly stores things and uses the space as a open shed! There is a gas bottle, an empty (I hope) petrol can, old furniture etc. etc. These are mainly against my garage wall, (making it awkward to read my gas meter) but under my bedroom, kitchen and lounge floor. I have mentioned my concern to the offending person in the past but without knowing for certain what my rights are I have not taken it further. Thank you for any advice.

I am not sure the layout of the premises, are you part of a block of flats although you appear to be independent, and is the areas below you part of the common areas of the flats. You could contact the local fire and rescue service, for a fire safety inspection of your home and if there was any problems they may be able to help. You could also try to contact the owners of the offending premises or local environmental health who may be able to help but quite often there is little you can do other than persuade the offenders to mend their ways. 

http://www.firesafe.org.uk/uk-fire-rescue-services-details/