If I have underlay and gripper down, what advantage is there to replacing  it?

New underlay not only makes the carpet feel more luxurious underfoot but will improve the life of the carpet.

"Cheaper carpets require a good underlay, Good quality carpets deserve one"

Will I have any work done to the subfloor if I have cushion floor vinyl fitted?

More often that not, Yes. because if the nature of Vinyl floor coverings. A good subfloor will result in a smooth flat final finish.

What sort of guarantee will I get with my new floor covering?

many different guarantees are given by the manufacurers, dependant upon product, specification of sub floor preparation and underlay type etc...

Q. 

Is there anything I can do about my rugs and mats sat on top of my carpet (or hard floors) to stop them creeping or moving?

A. 

Yes, there is a product we sell known as 'Rugstop' which applies to either rugs or mats on a variety of surfaces, that eliminates any creeping or movement.