Is this property in a clutch of properties where you clean within a tight-nit community?
If not, I wouldn't worry about slander, nor incurring expensive fees for solicitors letters, which are pretty pointless anyway, IMO. I've had a few myself in the past (nothing to do with work), and they just made me smile; a solicitor can't do anything; they just understand the law (or they should).
Reporting it to the police could be a good idea, if only to see what they have to say.
I fell out with a lass over a broken wishing well and she threatened to 'lose me customers', but I lost none. All her neighbours disliked the woman because she is mad as a fruit cake and agreed with me that what I did to recompense the lady was more than fair.
In fact I was going to buckle to her demands (it's awkward when you're not 100% sure you're in the right), and her immediate neighbour told me not to be stupid; that was good enough for me.
I personally wouldn't worry.