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Looks like I'm left with no option but the small claims route.
He did come over last week, unexpectedly, & agreed to take the cement, repay what he owed me & finish the path this week. When nothing happened in the next few days, I sent him an e-mail, followed by a text a couple of days later when no reply, asking when he'd be round (so I could be here) & when he'd be paying the refund. A day or so later I received another tirade, by text, saying I was "doing his head in" & I was taking him for a mug (eh ?), & that he'd sought advice & was happy to go "down the legal route".
Saw Citizens Advice - useless - & called their advice line & was told I had to send him a letter giving him 14 days (what !) to reply. I gave him 7. Then, when that's up next week, I have to write *again*, giving him 30 days else I'll go the small claims route. Meanwhile he's laughing all the way to the bank … & the path's *still* not fully finished ...