Can I withhold payment until render cleaners put right deck damage?

[Badgerstmary]

I had the render on my house treated recently. My house looks very clean but they spilt their chemicals on my deck. We now have a blotchy deck where the chemicals have bleached the wood it touched. They tried to wash the chemicals off at the time with a power wash but this only spread the chemicals out. The affected area is approximately 80cm along the whole wall.
We then cleaned and oiled the whole deck as normal but the areas with chemicals
on stayed the same bleached colour. The company had said that would sort out the problem but it made no difference.
The company are hassling me to pay them before sorting out their mistake. Where do I stand on this as I do not wish to pay until they have fixed my deck?

Get a quote to repair the decking and deduct this from what you pay for the render.

Thanks. They are currently talking to their decorator friend & saying he will come & fix it, but want their money 1st.

Don’t pay until they fix it. The job isn’t complete yet.

Write a letter setting out a reasonable deadline for them to fix the decking, after which time you'll be forced to bring in your own contractor, the cost of which will be deducted from their invoice.

As long as you've given them a fair opportunity to complete the job, and the quote is genuine and reasonable, they're unlikely to have a successful claim against you in civil court.

(not a lawyer)
It's not clear the amounts involved but you can only really withhold disputed amounts not the whole amount.
So if (say) the rendering job cost 10k and it's a single board of a softwood deck that's damaged then you couldn't justify withholding more than a couple of hundred of that.
However if the render job was just a patch costing 150 and it's 50 linear metres of hardwood decking needing replaced then actually there's nothing undisputed in what they say you owe.
You can't just withhold the whole amount by default as leverage.
I would recommend discussing how much to pay now and what to pay as a balance with the company. If it is a large company there's a chance the payments team doesn't know about the issue. If it's a small company they may just need their cashflow

Don't pay. Put it in writing that you expect the damage to be rectified by X date and if it isn't you will sort it yourselves and deduct the cost from their bill.

Have they admitted responsibility in writing?