Decorators doing a poor job

As I understand it - a written quote is set in stone and should be the final cost.
An estimate is merely a guess at what the cost might be.

I’d be inclined to send payment (cheque or bank transfer) for the original amount and never use them again.

Speak to Trading Standards at your local council. Even if they are not very helpful, you only have to say to this useless cntractor 'I have spoken to Trading Standards and they have advised me...........

DD did this and it worked a treat.

A quote is a legally binding contract Lily as opposed to an estimate so if you have a written quotation for the work, that is the amount you're legally obliged to pay.

An experienced painter and decorator, having seen the room to be decorated should have been able to quote taking into account everything required into account.

Some years ago we had renovations done to a front porch, the builder gave us a written itemised quote which we agreed to. As the work neared completion he told us the he had underestimated the price of the roof tiles and tried to up the price by a few hundred pounds, he got quite bolshy but we refused as it was his own fault for not checking the prices of materials properly. He did a decent job but wasn’t happy. We later found out he had done the same thing when fitting a large curved bay window for an elderly couple we knew, he had put them under pressure and they had reluctantly handed over the extra money.
As others have pointed out a quote is binding and different to an estimate, it’s up to the person doing the job to work out the costings accurately before agreeing a price. Im sure they win on some jobs and lose on others. Different if you ask for something to be done in addition to the original quote that would be an agreed add on but they should honour the price otherwise.

Did he do more hours than he thought he would have to do? Sometimes the amount of work increases when problems are found that would not have been evident on the first visit, you cannot expect more hours for no payment.

I would pay, if he sues and wins it will cost a lot more.

In Scotland I believe you are entitled to pay them the quoted price if you have it in writing and there was no mention that it might take longer or entail more materials than their estimate covered.

I have no idea what the position is in England, Wales or N. Ireland, but Citizens' advice ought to know.

www.citizensadvice.org.uk/consumer/getting-home-improvements-done/problem-with-home-improvements/#cant_come_to_an_agreement

General gist is you shouldn’t pay more than was agreed, if it was a quotation not an estimate. The trader should have priced it up properly. Quote the Consumer Rights Act 2015. If the trader is simply asking more, don’t pay.
But if you agree he has a good reason eg discovered a problem he couldn’t have anticipated, which took longer or needed extra materials, you might want to negotiate. Or go to dispute resolution.