David Cameron tweeted it and the Telegraph published the letter on the front page, listing 5,000 businesses who endorsed the Conservative Party in the General Election, many of which weren't businesses, weren't supporting the Tories, were repeat entries, or were individual employees of businesses who were incorrectly presumed to speak for their employers.
The "5,000 businesses" who signed the letter included charities (that aren't businesses); businesses that didn't actually sign it (many, many of these); local Conservative clubs; the same businesses listed repeatedly — "party members, candidates, cronies, retirees, volunteers, barristas, funeral parlour consultants, people who never signed it, people who have asked to be taken out, dissolved and liquidated businesses, people who responded four times, ghosts and the neighbour's dog."
The Telegraphy is often called the "Torygraph" for its partisan support of the UK Conservatives.
The letter itself originated with Conservative Campaign Headquarters, though this was not mentioned in the Telegraph's version of it.
If you want to know how actual small business owners in the UK feel about the election, here's some actual research on the subject: tl;dr: 21.7% support Labour, 20.9% support the Tories; 9.4% support UKIP; 7.7% support the Greens; 5.2% are Libdems; 4.1% are voting Other; and the rest (31.0%) are undecided.
UPDATE 20:00: I am beginning to have very serious doubts as to how many of even the legitimate businesses on the list actually signed anything. Aurum Solutions have issued a statement. Their sales director received an email from Brady "and recalls clicking on the link to find out more". That's it. He does not recall signing anything and denies strongly providing any information about the company. Could it be that this was merely an aggressive piece of spamming, where database entries referring to people and their workplace were signed up to this shambles at the mere click of the link?