I never said to invoice us..
You didn’t actually tell him to invoice you, no.
But neither did you tell him not to when it was suggested. You didn’t object or question them when they said they wouldn’t charge you anywhere near as much, -and you then went ahead to accommodate him in doing the work. In that way you entered in a sort of verbal contract by not objecting at any stage to the work he was doing. You did (albeit jokingly) say ‘add it to the invoice’ several times, further endorsing that verbal contract to them.
Now, because you failed to agree any terms whatsoever either at the beginning or any time along the course of his work, you have to wait to receive the invoice knowing you allowed yourselves to get into this mess without ever questioning or complaining about the shoddy work.
I know it’s daunting to complain about somebody’s work under these circumstances but you do have to speak up for yourself OP, and there are ways you could have done it kindly without falling out. eg Oh look you’ve missed a bit on this one again but never mind I’ll just redo it quickly myself so we can get finished a bit quicker. Or you could have said…yes add it to the invoice hahahaha but by the way, how much is this going to cost us because we never actually ever agreed a quote (or quality standards or time frame) at all did we.
Let’s hope you are pleasantly surprised at the £ amount they charge and can remain friends, instead of being really upset.
The ideal scenario would be that they only change for any materials, not for their time so that you can say a massive big thank you and arrange a small thoughtful gift and put it down to experience. At least you will have made an elderly gentleman feel useful again. But I’d say to be prepared for anything.
Good luck on Tuesday !