If a plumber did a really good job of fitting a shower in my house I would recommend him to my friends, it wouldn’t make any difference to me if when he'd finished he asked me to recommend him.
When people recommend a good tradesmen to a friend they don't do it to help the tradesmen, they do it to help the friend.
To increase the amount of word-of -mouth recommends you should do 2 things;
1) Do quality workmanship (nobody recommends a cowboy)
2) Don’t let the customer forget your name, (people might remember what a great job you did of cleaning thier suite, but can they heck remember your name) you do this with reminder cards and other mailings to keep your name fresh in their minds.
In the USA the give cash incentives to encourage recommends, this might work with commercial accounts but I don't think the British respond well to this. They recommend you because you are good, not to get a £5 backhander.
Mike
Mike