A restrictive covenant should be straightforward to enforce but will fail if it's too vague or too restrictive. The simplest contractual agreement would be a member of staff agreeing to pay a £500 introduction fee for any works gained during the course of their employment with you and that any works gained would, for the purpose of definition, include any work within 1 mile of either your office or their normal place of work within a 6 month period of their employment or effective termination date of employment (EDT) as appropriate.
The threat would be that a single claim would be enforced for each breach although I would have this as an implied term.
In enforcing this claim, the court would look to the temporal and geographical nature of the covenant to meet the test of "reasonableness" as defined by the Unfair Contract Terms Act (UCTA) and the claim would, following the normal channels for pursuing a debt, be through the small claim procedure. The advantage of that, as we all know, is that there are going to be no crazy legal bills to meet from the other side :-)
Two advantages in my opinion (and it is only my opinion, an LLB qualification doesn't make me a lawyer by any stretch of the imagination) of the above wording - it puts a monetary value on the breach which creates a reality for potential miscreants and it offers a negotiating value for someone who wants to go their own way.
If I found out that one of my staff had done this, I'd also be inclined to point out to the customer that I regretfully accepted no liability for a former member of staff who would be operating from that point on without my company insurance or support. It's highly unlikely for me at the moment since I don't currently do domestic although I'm about to have another tentative bash at the market.