There are a lot doing it, not only English people, but also German, Dutch, Swedes etc.If your using the internet to research how many are doing it then don’t assume the internet shows a balanced amount, its more normal for people to advertise locally, usually papers stuck to lamp posts or small notices in shops etc etc, people in Spain including the Spanish rarely use the net for business advertising and there are a lot of jobs being offered like this in Spain, regulations in Spain and their enforcement is worse than the UK, so anybody can set up a business and dog sitting is very easy to do without much outlay, which is due to the current crisis and people desire to stay in Spain, walking and sitting someones dog is an attractive source of income for the unskilled and there are a lot of unskilled people ruining the businesses of professionals, especially at a time when people need to spend less.
There always was cowboys doing work in Spain, since 2007 this has increased a hell of a lot, you’d be surprised how many waiters worked as skilled craftsmen on building sites in the boom, hell the guy who owned La Canada, Marbellas biggest shopping centre with 280 outlets was a sheep farmer for 20 yrs until he sold some land, suddenly became a builder who amassed enough money (Tens of Millions) to build his own home, a replica of the USA White House, then got done for illegal building and lost a lot of cash, he’s probably a financial advisor now or more likely a dog groomer to his rich clients.
Even retired people with there own animals have sprouted up as this is a viable way to supplement their pensions.
http://europe.ninasnanniesforpets.co.uk
http://www.housesitworld.com/europe/
http://www.myservicesdirectory.com/Category/pets/Kennels-and-catteries/2.html