The cable is just as important as the aerial (and any amp or splitter, if fitted) in providing a decent signal on your set. The cable should be changed as a matter of course if any work is undertaken on the aerial, unless you are sure it is sound and of good quality.
A typical cable run (from the roof) is about 15m. Bear in mind that it is preferable to order a bit too much, than a piece that’s a bit too short….. Any extra cable can be made into fly leads, which, I guarantee, will be better quality than those you already have.
For (up to) 30m of this cable the P&P is cheaper for this item if you phone the order through !