Passing off is a tortuous action taken when a competing company uses the goodwill and reputation of another to gain an unfair commercial advantage.
To succeed in a claim of passing off you must show that there has been a misrepresentation to the public that is likely to cause confusion over the origin of the goods. What does this mean in real terms?
A Mark needs to be shown as distinctive to set it apart from other goods and services, the more generic the term, the harder it is to prove distinctiveness. On top of this a substantial number of consumers must be deceived (Norman Kark Publications v. OldhamÂs Press Ltd  1 W.L.R. 380) but the likelihood of confusion need not be so high as to be required to place the objects side by side and compare every single difference and similarity (Seixo v Provezende (1865-66) LR 1 Ch App 192)
A court will need to see evidence that your company has been trading for a long period of time and therefore gained goodwill over the years. To prove this a simple printout from companies house will show when your company was registered and when the infringing company was registered. A Website or online presence will also be of aid in this claim, as this will show a presence to the public of being able to purchase the item and dates of how long the goods have been on offer.
A great way to show reputation and goodwill is by letters of businesses recommending your product or stating how much they like your product. In terms of websites many customers leave reviews of the product for others thinking of purchasing the product to sway their judgment.
To show Misrepresentation is a difficult matter, to succeed it must be proved that the offending product is likely to confuse a substantial part of the public. A letter from a previous customer stating that they were confused by the two products and they believed that the infringing product was one that belonged to you. Relevant advertising and packaging statements of the infringing product will be invaluable, as this may indication a misrepresentation to the public. Photographic evidence of the products and usage i.e. on the shelf of a shop will aid in showing similarities and likelihood of confusion to the public.
In recent cases, the allowance of surveys of public confusion has come under fire. It would seem that a public survey is not a piece of admissible evidence to the court (Interflora v Marks & Spencer  EWHC 273 (Ch)) and later reinforced in Zee Entertainment Enterprises Ltd and other companies v Zeebox Ltd  All ER (D) 263 (Jun).
If you can prove all of the required evidence and prove that the infringing product is trying to confuse the public and gain an unfair commercial advantage then you will succeed in passing off.