At the top of the letter put in capitals
LETTER BEFORE ACTION
Then something along the lines of
Dear Mr and Mrs scammer,
this is a letter before action for the recovery of £43 for the cleaning of the windows of ********* their address............ twice.
After numerous attempts to collect the sum owed and numerous excuses by yourselves, I would require the full sum paid into the bank account .............. by end of 7 days from receipt of this notice. Failure to do so will result in a claim being filed with the small claims court, for the outstanding amount and all costs incurred therein.
Yours Sincerely p1$$ed of window licker......
You must however be prepared to file the claim... it is easy enough to do online..
Oh and send it by email and post, but get proof of posting (free) from the post office.