You've (perfectly reasonably) posted within the 'Law' section but I think that an alternative approach might be better. (See my last paragraph, below).
The only legal process for dealing with the situation is to write a formal letter to the window cleaner, demanding either the return of your key or £19 to pay for a new one. That letter should clearly state that, unless the matter is resolved within a specified period of time (I suggest 21 days) you will commence legal action. If you then don't get the key (or the money to pay for a new one) you'll have to start court proceedings via HM Courts Service's online claim system:
https://www.moneyclaim.gov.uk/web/mcol/welcome
That means that you'd have to pay the court fees (although the court could then add those fees to the money the window cleaner owes you). While that's the 'official' way to do things, you might decide that it's not worth it for £19.
In my opinion, a far better approach would be to tell the window cleaner that, unless he pays up, you will contact your local newspaper with your story about his worker urinating on your property and his failure to return your key. The threat of adverse publicity for his business is probably far more likely to get him to pay up than any threat of court action.