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Ask for a comprehensive quotation that includes a scope of works. As part of the process it isn't unreasonable to ask for photo documentation of all work carried out, especially stuff that won't be visible after work is completed. I do this with almost all work I carry out for arse covering reasons and provide it to clients in a report once work has been completed, as seeing the layout of studs / hidden pipework / wires down the line can save them a lot of hassle.
If the work has already been completed ask for receipts etc for materials bought, accepting that there will be a markup. Again this is something I'm happy to do but many tradespeople may be reluctant to do so.
What steps do I need to take to deal with tradesmen taking the piss with claiming for work they haven't done.